Simon Cornelius, Dan Jackson and Doug Longtin, Baker Hughes

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T
he modern refining industry is faced with many challenges, but one of the
most common processing challenges referenced is the ability to process
difficult heavy crude oils successfully and profitably. Heavy crude oils are
typically defined as dense and viscous, with an API gravity between 10 –
20˚. Heavy crude oils (<20˚ API) are well known in the US and have been a desalter
challenge from as early as 1990. The subject has been addressed at the American Fuel
and Petrochemical Manufacturers (AFPM) Q&A conferences, where desalting
operational and mechanical techniques for heavy crude oils have been discussed. The
key issues many refiners face are system limitations, or when the original design
criteria trails market economics while supplies of heavy oil are used.
HYDROCARBON
ENGINEERING
Reprinted from September 2012
Approximately 70% of the heavy crude oils produced
come from North and South America, with significant
quantities being produced in the US, Canada, Mexico,
Venezuela and Brazil. The Middle East follows with 11%, then
Asia, Europe, Russia and CIS, and finally Africa. The Canadian
oilsands and the Orinoco Belt extra heavy oil are the largest
sources of non-conventional oil production today and are
expected to remain so in the future.1
Operating challenges
Desalting operations have been the process engineer’s
challenge since crude oil was first washed with water and an
emulsion formed. Yet in 1911, when electrostatic precipitation
was applied to liquid/liquid separation and the first patent
was awarded,2 new equipment added another desalting
variable.
Desalting variables3 often mirror many integrated layers
that relate to the electrical desalter as the primary device in
the refinery to remove undesirable components. However, as
oils increase in density, the challenge sometimes defies logic
as the density approaches that of water. Thus, the process
engineer must take advantage of the many available strategies
afforded to the industry:
Mixing
To remove contaminants from the crude, it is necessary to
contact the wash water droplets with the contaminant
directly.
Chemicals
Demulsifiers or emulsion breakers are specialty chemicals
used to aid in the separation of oil and water, to condition
solids and to achieve acceptable water coalescence while
maintaining unit performance for salts, basic sediment and
water (BS&W) and contaminants.
Mud washing
Heavy opportunity crude oils contain greater quantities of
contaminants, such as sand, clay, iron sulfide, iron oxide and
other solid materials. These materials settle out in vessel
bottoms forming sludge or ‘mud’. These solids need to be
removed on a controlled regular basis.
Slop or recycled oil handling
Slop oil handling is one of the most common sources of
upsets and poor performance at the desalter; challenge crude
and harmful slop (lesser quality) are not compatible.
Stokes’ law
Tank farm management
The speed of separation is inversely related to the viscosity;
emulsion separation is slower.
Desalter upsets are often related to crude quality issues;
crude quality is impacted by crude types, geological
formations and world economic opportunity. In the tank farm,
these issues may be non-events or complex problems left to
be manifested at the desalter.
Temperature
In general, raising the temperature will improve the oil/water
separation in the desalter because the viscosity of the
hydrocarbon decreases as the temperature increases.
However, there are negative impacts when raising the
temperature (e.g. asphaltenes). These can become unstable as
the temperature rises and the precipitated asphaltenes
collect at the oil/water interface to stabilise the emulsion
and cause oil undercarry.
Wash water source
Water sources are a concern because of possible
contaminants. Generally, stripped sour water or overhead
accumulator water is desired. Changes in wash water sources
can have a dramatic impact on the desalter performance.
Heavy crude oils require higher water rates.
Wash water injection
When injected before the mixing device and with the
optimum mixing energy, contaminants may be washed or
removed from the oil. As oils become heavier, wash water
rates are increased to improve contact and water population.
Desalter level and control
This is one of the most important variables that refiners seem
to struggle with when processing heavy oils. Each desalter
manufacturing design recommends a target operating level.
Level controls have progressed, but tryline (swing valve)
monitoring is and has always been used to ‘see’ inside the
vessel.
Reprinted from September 2012
HYDROCARBON
ENGINEERING
Brine quality
Originally, desalter designs targeted only oil quality
performance (salts and BS&W). Today, waste treatment or
biological processing usually manages and controls this
aspect.
Equipment limitations
Equipment may not be designed for heavy crude.
Performance monitoring
All systems require thorough detailed operating knowledge to
monitor and judge performance. System key performance
indicators (KPIs), salts, dehydration and solids are used to
judge downstream fouling, corrosion and equipment
performance to enable the process engineer to evaluate and
judge system performance.
Strategies
The refining industry has long addressed desalting as a single
unit within the crude complex. Issues occur at the desalter,
meaning performance is not being met or an upset has
occurred, which requires some aspect of the aforementioned
strategies to be adjusted. The operator has limited options to
adjust or fix the problem.
In the mid 1980s, Baker Hughes began addressing heavy,
difficult crude oils and those containing slop oil contaminants
using a broader approach. The tank farm operations were
surveyed and understood to judge impacts, such as blending
of each component.
Crude management and processing is traditionally
thought of as a continuous process. However, in reality each
cargo of crude, especially today with opportunity crudes
(Canadian, South American or new production crude oils), is
actually a singular parcel that needs to be judged and
managed for compatibility to minimise desalter upsets.
Crude handling encompasses a wide range of activities and
varies from one refinery to another as a function of equipment
(receiving methods, tank configurations, etc.) and oil movement
practices. Some refineries have tanks with floating suctions;
some have fixed bottom discharges. Some have continuous
mixing, while some either do not have mixers or have mixers
that do not work. Some refineries fill/still/water draw while
others may operate on ‘running gauge’ (live tanks). Even water
drawing practices (if practiced) can vary dramatically. For
instance, one refiner may instruct operators to pull clear water
and stop at ‘black water’, a method that inherently leaves behind
cuff and microemulsions. In contrast, another refiner may
instruct to pull all the way until the draw is ‘dry’ oil. Regardless
of practices followed, these interfaces and the microemulsions,
solids and emulsion stabilisers they contain impact desalter
operations if not recognised and properly addressed (Figure 1).
Feed contaminants manifest themselves as problems in
two forms: acute and/or chronic. Acute problems are abrupt
desalter upsets that can be related back to situations that stir
up or draw on tank sludge or microemulsions that have built
up over time. Such situations include tank filling and/or
running to low gauge. Chronic problems are more related to
the quality of the incoming crude components and are usually
indicated by high filterable solids and/or high BS&W. Chronic
problems frequently show up as either oily desalter tail
(brine) water or diminished dehydration and salt removal
performance depending on the operation.
Tank interfaces, sludge accumulations and recovered oil
reruns are by far the most common causes of deterioration in
desalter operation. In any operation, the ability to fine tune
the process to optimum efficiency is directly dependent upon
the variability of the feed source. The more variations
encountered, the less the operation can be optimised. If a
desalter is subject to wide variations in crude quality (with
regard to emulsion stabilisers), tightening up the mix and
other parameters can become very unforgiving. The Baker
Hughes Crude Oil Management™ programme takes into
account all components of the crude/recovered oil handling
system and dictates not only a thorough understanding of the
various systems but also a proactive role in working with all
refinery groups involved to align objectives. Crude Oil
Management (Figure 2) is a stewardship/management
programme that includes knowledge of oil movements, water
draw activities and oil recovery operations as they may
impact desalter operations.4 Thus, in order to manage desalter
reliability, several new proven tools and strategies are
available to address possible desalter limitations and improve
performance when processing heavy crude oils.
Desalter tools and keys
Pretreatment
Figure 1. Tank farm emulsions impacts.
Figure 2. Crude Oil Management programme:
desalter reliability.
Baker Hughes developed the concept of pretreating crude
receipts in tankage more than 30 years ago when processing
difficult crudes, slop or recycled oil. This concept has further
aided the processing of heavy, opportunity crude slates with
high contaminants (solids). Pretreatment allows a larger
operating window with challenging crude slates.
A basic concept of crude pretreatment lies in the fact that
all chemical reactions take place by molecular interaction,
which takes time. When a ppm level of demulsifier or wetting
agent is added within the desalter unit, the chemical has a very
limited amount of time to find and react with other ppm level
emulsion stabilisers. In contrast, when added onto crude
receipts (or recovered oils) ahead of tankage, the specially
designed treatment chemical has significantly more time to
locate and react with the target agents, thus making the
process more efficient and effective.
A critical component of crude pretreating is product
technology. Demulsification chemistry is complex. Building
effective heavy oil demulsifiers requires a complete
understanding of the functionality of all intermediates and
components. Performance enhancing demulsifiers are blends
of resins, imparting specific qualities to the oil (i.e.
dehydration, solids deoiling). The chemistry functions at the
interface and on stabilisers ranging from oil coated solids to
chemical emulsifiers. Formulations must address oil/water
partitioning factors, solids wetting capabilities and surface
tension. Most of these factors are directly affected by
temperature and pH, conditions that are drastically different
at tankage than at the desalter.
HYDROCARBON
ENGINEERING
Reprinted from September 2012
The XERIC™ formulations Baker Hughes developed have
proven to perform over a wide range of heavy crude oils: the
XERIC 7020 demulsifier, for example, has broad spectrum abilities.
The benefits of crude pretreatment are:
nn Stabilised desalter operation.
§§ Provides more residence time for ppm level chemicals
to associate with solids and emulsion stabilisers.
§§ Solids are preconditioned to be more easily water
wetted at the desalter.
§§ Eliminates sudden desalter upsets that can occur as
tank levels are pulled down to low gauges where
interfaces would otherwise reside.
§§ Reduces chemical demand at the desalter itself.
nn Sharper tank interfaces.
§§ Gives clearer definition between water and the oil
boundary layer.
§§ Allows the tank farm operator to pull more from the
water phase during the water draw procedure.
§§ Reduces the oil loading to the effluent handling
system.
nn Reduces tank sludge volumes.
§§ Acts over time to resolve microemulsions that make up
a significant portion of tank sludge volumes.
§§ Increases effective working volume of crude storage
tanks.
§§ Reduces volumes of sludge that otherwise have to be
processed or handled as hazardous waste during
periodic tank turnarounds.
Recycled oil and slops
Many times, refinery slop oils are returned to crude storage
with little regard to the potential impact on crude slates and
the desalter (Figure 3). Processing heavy opportunity crudes
can be very unforgiving when water or emulsions are mixed
with challenge crudes. Slop oil recycle should meet certain
criteria. Generally, the minimum quality specification is BS&W,
with <2.0% as a minimum. However, slop oils may contain
many species (amines, caustic, solids and emulsions) that may
or may not manifest desalter upsets. The Baker Hughes Crude
Oil Management programme addresses this to minimise any
impact.
Mix valve optimisation
Figure 3. Returning low quality recycled oil (slop)
to crude tanks when processing heavy crudes can
lead to major upset conditions in the desalter.
These two samples returned to crude tanks
(water, BS&W and emulsions) resulted in large,
unresolved desalter emulsions.
With traditional light crude oils, the mix valve was optimised to
achieve maximum salt removal. To accomplish this, the mix
valve was increased to maximise mixing efficiency and move salt
into the water phase. This was balanced against dehydration,
which tends to deteriorate as the mix valve increases.
The picture has changed for heavy crude oils (Figure 4).
With larger solids stabilised emulsions, the mix valve can
affect the brine quality as well as dehydration and salt
removal. Refineries are forced to reevaluate their priorities
when it comes to desalter performance and choose between
maximum salt removal and maintaining clean brine water.
Since most wastewater facilities are not equipped to handle
brine oil undercarry or oil coated solids, salt removal
typically suffers for the sake of maintaining clean brine. In
addition, the inherent variability in feed quality that comes
with processing heavy crude oils requires the mix valve
setting be reevaluated on a daily basis to avoid emulsion
band growth and upsets.
Temperature
Figure 4. Mix valve optimisation.
Table 1. The impact of the wash water rate on
coalescence and dehydration
Settling time
3% water
5% water
7% water
9% water
20 min
1.2%
3.4%
4.5%
8.0%
30 min
1.7%
3.7%
5.5%
8.0%
Desalted crude
BS&W
1.2%
0.8%
0.8%
0.2%
Reprinted from September 2012
HYDROCARBON
ENGINEERING
Heavy crude oils, by nature, typically contain higher
percentages of asphaltenes. Many of the heavy crudes,
primarily Canadian crudes, have to be cut with lighter feed for
transportation. Lighter feeds are often high in paraffin
content, which causes asphaltene precipitation (instability),
when mixed with heavy oils. When asphaltenes precipitate in
the desalter, they tend to accumulate at the desalter interface
and stabilise the emulsion, causing it to increase in size.
Asphaltene precipitation is impacted by several variables, but
data shows asphaltenes tend to precipitate with increasing
temperature.
This challenges the long held philosophy that higher
desalter temperatures are better. Certainly, from the
standpoint of viscosity and specific gravity there are
advantages to operating the desalter up to the limit of the
equipment. However, with unstable crude, the refinery is well
advised to keep the desalter temperature in check, or risk
emulsion growth and the possibility of oil undercarry.
A field study conducted to show desalter temperature
versus brine quality demonstrates this impact (Figure 5).
Asphaltene precipitation was occurring during higher unit
temperatures. By decreasing the desalter operating
temperature by ~11 ˚C, the refinery was able to clear the brine.
Wash water rate
Wash water rate is a critical variable in desalting. The primary
goal of water wash is to provide dilution of the salt and solid
contaminants. It is well known, at constant dehydration and
mixing efficiency, that salt removal will be almost linear with
the rate of wash water. Thus, increasing the wash water rate
will have a positive impact on salt and solids removal. In
addition, a higher rate of water will increase the droplet
population and improve coalescing and dehydration. This is
very important and especially true with heavy crude oils.
Given that heavy oils are generally more viscous, mixing
efficiency and dehydration are reduced, making the maximum
water wash rate critical for salt and solids removal. The results
of a study conducted on heavy oil and the impact of the wash
water rate on coalescence and dehydration are shown in Table 1.
When planning to process heavy crude, refineries need to
make provisions for higher wash water rates. While 4 – 6 vol%
water is typically sufficient for lighter crude oils, 6 – 9 vol% is
recommended for heavy crude oil with an API gravity of <26˚.
Water availability, hydraulics and water residence time must all
be taken into account when considering this modification. One
option that many refiners have chosen is to recycle brine water
back to the mix valve in an effort to improve dehydration and
salt removal. Recycling water will not improve salt removal
from the standpoint of dilution; however, it will improve
dehydration as a result of the increase in droplet population.
New demulsifier development
Baker Hughes has been involved in chemical demulsifiers since
William S. Barnickel received the first patent for oil-water
emulsion resolution in 1914.5 Historically, successful
demulsifiers, controlling desalter emulsion resolution, have
allowed refineries to process a wide range of opportunity
and/or heavy crudes. Success has been built on chemistries
providing acceptable performance for:
Figure 5. Desalter temperature versus brine
quality: lower operating temperature resulted in
improved brine quality.
nn
nn
nn
nn
nn
nn
BS&W.
Salt content.
Water coalescence and clarity.
Interface quality.
Chemical rate/range (economics).
Solids control.
In 2008, new research and development improvements
provided a series of new heavy oil demulsifiers (including the
XERIC demulsifiers) to compliment the tools and strategies
already in use. Immediate impacts were witnessed. These new
demulsifiers improved brine quality and solids conditioning
(Figure 6).
Emulsion control/resolution
Emulsion resolution is often compromised for heavy crude
slates. Pretreating with the correct chemistry application
allows better desalting control (Figure 7).
Mudwash solids control
Due to the processing of heavy crudes with greater and
greater solids, new application techniques have been
developed to enhance and allow the refinery to manage
solids better. For many years, mudwashing the desalter has
been the centre of an ongoing debate regarding best
practices. Typically, with heavier crudes and more and more
solids, new techniques have been applied during mud washing
to allow solids control improvements.
Figure 6. Midwest refinery heavy Canadian crude
oil: desalter brine quality improvement.
Rapid upset recovery
For each heavy crude processing challenge there are proven
strategies that can be implemented to meet it. Yet for
HYDROCARBON
ENGINEERING
Reprinted from September 2012
refineries that are designed to process only light, low solids
crudes, these strategies can mean significant process and
equipment modifications that require resources and capital
expenditure.
Many refineries lack the facilities necessary to maintain
stable, trouble free desalter operation. As a result, Baker
Hughes has developed novel chemical and application
technologies to compensate for inadequate facilities,
featuring procedures to bring the desalter under control
during an upset condition. The new products, featured in the
XERIC product line, are specially formulated to remove oil
from solids and resolve the most difficult emulsions. The
versatile demulsifier/wetting agent combinations are used to
collapse a growing emulsion band by wetting and releasing
solids where they can be removed with the brine. Use of the
mudwash system is a key aspect of this specialised treatment
approach.
The application strategy has led to permanent injection
facilities at several refineries that use these products and
application techniques. In a recent example at a midwest
refinery, USA, tryline samples were taken approximately 1 ft
apart. Before the addition of the new XERIC demulsifier, the
bottom five trylines contained emulsified solids, water and
oil. Most critically, the refiner was unable to maintain clear
brine as a result of the high solids in the feed. A specially
formulated XERIC heavy oil demulsifier was injected at a
targeted rate to collapse the emulsion band and improve
brine water quality. After a four hour treatment, the emulsion
band was reduced substantially, leaving clear brine and water
with oil free solids in the bottom two trylines. The rapid
upset recovery treatment had a lasting effect, resulting in a
‘reset’ of the desalter conditions.
problem manifests itself by lowering soluble oxygen in the
aeration basins, inhibiting new bacterial growth and creating an
environment where filamentous bacteria can thrive.
The results of these conditions are poor settleability in the
secondary clarifier and total suspended solids carryover. Loss
of nitrification under these conditions generally follows,
leading to potential permit violations.
Brine treatment strategies
Baker Hughes recommends treating the desalter brine by
diversion to a secondary separation vessel, along with the
addition of specialised chemical products to assist in
separation. Product testing is necessary to select proper
demulsification aids and reverse demulsifiers. Certain
metals precipitants can also be added to enhance heavy
metals removal. A range of XERIC products has been
developed for reverse demulsification specifically for heavy
crude oils.
Brine quality and downsteam
impacts
Brine quality has become a major area of concern as refineries
continue to process various discounted heavy crudes. Poor
crude blending practices and incompatibility can destabilise
asphaltenes, which then end up in the brine. Mud washes and
desalter upsets can be one of the most difficult streams for
the refinery wastewater plant to process. The two major
issues associated with poor brine quality are stripper fouling
and high organic loading.
Benzene/stripper fouling
Figure 7. Heavy crude oil desalter trylines. Better
emulsion control equals better mudwashing and
less wastewater treatment issues.
Free oil, emulsions and solids can foul stripper trays/media,
significantly reducing stripping efficiency and potentially
creating a benzene release violation.
High organic loading
Insoluble chemical oxygen demand (COD)/biochemical
oxygen demand (BOD) will contribute to high organic loading
to the wastewater plant. Stable reverse emulsions and oily
solids are the main bad actors. Oily contaminants in these
streams will usually not float or sink in the API separator. If no
other primary treatment equipment exists, this can
potientally pass straight though the primary system and end
up in the biological treatment area.
High organic loading can put significant stress on the
secondary biological system bacterial population. This
Reprinted from September 2012
HYDROCARBON
ENGINEERING
Figure 8. Primary treatment equipment: heated
cone bottom tank operation.
Secondary separation vessels
Secondary separation vessels have the advantage of increased
residence time (compared to that of the desalter) so the oil
emulsion is given more time to resolve into an oil layer and an
oil free water phase. This vessel also can be used to impound
the emulsion before it reaches the wastewater treatment plant
or benzene reduction equipment.
While there are many designs for such equipment, they all
share a number of common features. The water is generally
drained and sent to a floatation device. Solids are pulled from
the bottom and sent to a centrifuge, and oil is skimmed off
the top and sent back to slop. There are several methods for
monitoring the oil/water interface; these can be as simple as
using try cocks for manual sampling or level sensors tied into
automatic level control (Figure 8).
Deoilers and water quality improvement
Further treatment of primary vessel water draw is
recommended to remove emulsified oil. Various floatation
vessels are used for this purpose and have the following
advantages (Figure 9):
nn Effluent can be sent to the benzene stripper to determine
if it meets national emission standards for hazardous air
pollutants (NESHAP) conformance with little to no
fouling potential.
nn Reduces insoluble COD/BOD.
nn Reduces overall organic loading to wastewater treatment
plant.
nn Excellent point source control measure.
nn Treats only the emulsified portion of the desalter
brine.
Chemical addition
In order to improve the efficiency of separation in these
vessels, it can be useful to inject specially developed chemical
products into the desalter rag draw or effluent water. These
products include ‘normal emulsion breakers’ similar to those
Figure 9. Primary treatment equipment: floatation
vessel.
traditionally used in desalting and water soluble polymers, or
‘reverse emulsion breakers’. The purpose of these products is
to make a clean oil/water/solids split (Figure 10).
The reverse breakers are water soluble polymers and
coagulants, which are typically cationic in charge but can also
be anionic or non-ionic. They include inorganic coagulants,
such as poly aluminum chloride, which has been used either
alone or in combination with organic polymers since the late
1980s for this type of application. Most commonly used are
organic water soluble polymers and polymer solution
emulsified in carrier oil (emulsion polymers).
The water soluble polymers are easiest to use since they
do not require aging tanks or any complicated ‘make down’ or
injection equipment to function at optimum efficiency. The
water soluble materials typically have molecular weights
(MW) below 1 million, since higher MW polymers are either
too dilute or too viscous for easy handling. Chemically, the
water soluble materials include polyamines, poly DADMAC,
and other such cationic polymers.
High molecular weight emulsion polymers have proven to
be very effective in clearing up the water in these secondary
treatment vessels. The MW of these polymers is in the millions
and can be more than 10 000 000. The disadvantage of these
emulsion polymers is that they require specialised equipment to
‘make down’ properly. Generally they must rapidly be added at
approximately 0.1 – 2% to dilution water with high mixing, so
that the emulsion will invert. If the ‘make down’ is not proper,
then the polymer will form large gels, which can deposit in
injection and process equipment.
While inorganic coagulants, solution polymers and emulsion
polymers have been used for years for such applications, there
was still a need for a product with the handling ease of a
solution polymer and the effectiveness of high molecular
weight emulsion polymers. New XERIC dispersion polymers
have been developed to meet this need.
Conclusion
Heavy crude oils will continue to be processed, both today
and in the future. Providing a proactive approach to managing
desalter impacts affords a refinery with reliable and
predictable desalting performance. One tool or solution does
not fit all. Successful desalter strategies address crude oils
beginning with crude oil tank farm receipts and managing oils,
and processes throughout the refinery, including recovered oils
and brine waters. The process requires diligence, innovation
and chemistries for success.
Notes
Crude Oil Management and XERIC are trademarks of Baker
Hughes Incorporated.
References
Figure 10. Untreated brine and treated brine with
increasing coagulant dosage.
1. Hart Energy Consulting, 'Heavy Crude Oil: A Global Analysis and
Outlook to 2030', November 2010.
2. Petroleum Rectifying Company of California (Petreco), U.S. Pat. No.
987,115.
3. KREMER, Lawrence N.; Baker Petrolite; 'Desalter Tutorial: Best
Practices for Best Results,' AIChE Meeting 2002 Spring AIChE
Meeting New Orleans, La.
4. BROWN, William M.; Baker Hughes internal documentation –
'Crude Oil Management & Pretreatment – Successful Techniques to
Improve Desalting and Address System Operating Limitations'.
5. ZETLMEISL, Michael J.; Baker Petrolite History 2009 internal
documentation, US Pat. No. 1,093,098 the first chemical
demulsifier.
HYDROCARBON
ENGINEERING
Reprinted from September 2012
Heavy Canadian crudes.
Multiple processing challenges.
The implementation of a
XERIC heavy oil program
more than doubled the
Canadian crude feed rate
of one refiner.
Canadian Feedstock
20,000
Solving problems when you process heavy crudes
means having a partner with the right knowledge.
To help one U.S. refiner overcome difficulties processing
a heavy Canadian crude (19° API), we recommended
© 2012 Baker Hughes Incorporated. All Rights Reserved. 35766
our XERIC™ heavy oil program. Together with a heavy
oil demulsifier and solids conditioning aid, we identified
a suitable blending aid using proprietary asphaltene
stability testing. That combination decreased the
rag layer while improving effluent water quality and
dehydration efficiency. As a result, the refiner earned an
additional USD 3-4 for each barrel of heavy oil processed.
BD
Improve your profitability,
performance, and throughput.
15,000
10,000
5,000
0
Before
Baker Hughes
treatment
After
Baker Hughes
treatment
bakerhughes.com/xeric