Oily wastes are a normal byproduct of many operations
carried out by the oil industry between well and pump. The path
starts at production and continues on through transport and refining. Although the waste
quantity is small compared to the industrys overall
output, the size of the industry means that these oily wastes
can add up to a considerable problem. One concern with these
wastes is the loss of useable volume in crude oil tanks or
emergency lagoons due to the volume taken up by the waste.
Another concern is the environmental hazards that the waste
may pose. Direct disposal of these wastes without prior
treatment is normally not possible, but, apart from the
requirement for some form of treatment, todays crude oil
prices often provide an additional incentive for treatment, as
oil wastes are a valuable resource that can be recovered if
In this context the decanter, or solid-bowl centrifuge, is a
very versatile instrument and key component of most advanced
oil waste treatment plants.
Due to the variability of the oily waste as feed product
(because of different crude properties, different histories and
the origin of the waste), treatment systems should be tailored
to each projects needs, rather than
being a standard unit put to work under all kinds of
conditions. The decanter centrifuge along with ancillary and
additional equipment, can easily be adapted to such varying
Approaching product and treatment.
Oil waste is not a precisely defined product, but it varies
over a wide range with regard to its composition and
properties. The hydrocarbon content is typically at the
heavier end of the spectrum, but this will depend on the oil
field where the crude was produced. The waste may also include
light hydrocarbons, or it may represent only a certain fraction
of the range. Water is often present, but not always. To offer
a common example, the solids could be sand, rust or organic
matter. There is also often an emulsion present as a fourth
component that comprises hydrocarbons, water and captured fine
solids. Some examples illustrate the variety of products that
are collectively addressed as oil sludge:
Waste oil drilling mud
Oilfield pit sludge waste
Storage tank bottom sludge
Emergency lagoon sludge
Oily sludge from refinery effluent treatment.
Similarly diverse are the primary treatment targets,
depending on the project specific background. While, in one
case, the only target may be to achieve a product that can
safely be disposed in a landfill, another project may require a
final product that is good for incineration. Then there might
be a project that requires good quality oil for recycling into
the refining process, or needs a good
water phase for re-use in the process. Naturally, the product
properties, together with the primary treatment targets, will
determine the basic process steps, as shown in a few examples
in Table 1 and Table 2.
Due to this vast diversity, laboratory analyses and tests
are extremely important in order to determine the
products properties, the results that a particular
treatment method is likely to achieve in full scale and the
respective process requirements to run the treatment process
Other parameters that are required as input into the
treatment plant design include product and site data (including
customer regulations) relevant for explosion protection,
available heat sources and general installation
Operating principle of decanter centrifuges.
The solid-bowl or decanter centrifuge is the machine of
choice for treatment of oil wastes. While other products often
leave the option to use belt-filter presses or plate-and-frame
presses instead, those types of machines are not suitable for
oil waste treatment. Even at low oil contents, the sludge will
stick to the filter cloth and will eventually blind it.
Similarly, three-phase separation, where the solids are
separated from the liquid phase and the liquid phase is split
into oil and water, is impossible to achieve with presses.
Contrary to presses, which work on the principle of
filtration, decanter centrifuges are separating via the
principle of sedimentation by making use of the different
specific gravity of the oil sludges components. The
decanters operating principle is analogous to a
continuously fed sedimentation tank with a bottom scraper. In
such a tank, solids will settle on the bottom and oil will
float on top of the water under natural gravity, provided the
hydraulic retention time in the tank is long enough for the
separation to take place. This simple principle is also often
used in various places in the petroleum industry.
For fine solids and/or viscous hydrocarbons, the time
required to achieve separation under natural gravity may be too
long to be economical, or the separation result may be less
than desired. This is typically the case with oil sludges, and,
therefore, sedimentation is enhanced in the decanter centrifuge
by increasing the driving force by three orders of magnitude,
spinning the product so fast that centrifugal accelerations of
up to several thousand times g (= gravitational acceleration;
average = 9.81 m/s2) are created.
Using the analogy of the sedimentation tank again, the tank
may be rolled into a cylinder with the tank bottom becoming the
cylinder wall, and the water body and scraper being located
inside the cylinder. This cylinder, called the cylindrical
bowl, is rotated with several thousand rpm around its
longitudinal axis to create the required centrifugal
acceleration. Feed into this system is continuous via a fixed
pipe extending from the outside into the center of the
cylinder. In this system, the solids will move radialy outward
and settle on the inner surface of the cylinders wall.
Water will form a layer sitting on top of this sediment layer,
and oil (if present) will form a third layer further inward
(toward the center) on top of the water layer. The shape
of the bottom scraper will be changed to that of a screw
conveyor, also called the scroll, which will rotate with a
slightly different speed to the cylinder in order to convey the
sediment toward the outlet.
To one end of the cylinder, a truncated cone (also called
the beach) is added in order to block this end for the liquids.
It is over this beach that the solids are conveyed and
discharged out of the bowl. The other end of the cylindrical
bowl is closed with a head wall that has outlet openings with
adjustable weirs for discharge of the liquid. Further devices
can be added to the decanter centrifuge to separately extract a
second liquid phase if present. Fig. 1 shows a schematic
drawing of a two-phase decanter centrifuge.
Fig. 1. Schematic
drawing of a two-phase
Tailoring the decanter centrifuge to the task.
In order to be able to perform most efficiently and
economically under site conditions, tailoring the decanter
centrifuge to the project specific requirements is
required. This includes:
Materials of construction for wetted parts,
taking into account corrosion, abrasion, operating temperatures
and operating/maintenance regime
Wear-protection level and system, taking into
account abrasiveness of the product, the size and nature of the
solid particles contained in the product, the operating and maintenance regime and available
Liquid-phase extraction devices, in the case of
three-phase separation, taking into account the variability of
the content of both liquid phases, quality requirements for the
separated liquid phases and downstream plant/equipment
Explosion protection measures, taking into
account ambient conditions and product properties at operating
conditions and national and/or customer specific
Drive system for bowl and scroll, taking into
account product and process requirements and site
While these choices will only marginally affect the capital
expenditure for the decanter centrifuge compared to the cost of
the entire treatment plant, their effect on the operability,
availability and efficiency of the decanter centrifuge as the
core piece of the plant can be considerable. Therefore, special
care has to be taken that these options in all their breadth
are indeed available and considered in depth, and eventually
the right choice is made and implemented. Two examples for
decanter centrifuges are shown in Fig. 2 and Fig. 3 to
demonstrate some of the differences in design.
Explosion-proof (ATEX) two-phase
decanter with hydraulic scroll drive.
Explosion-proof (ATEX) three-phase
decanter with electro-mechanical
Examples for oil waste treatment plants.
The following two examples of oil waste treatment systems
are discussed in order to illustrate the aspects outlined above
and thus provide the link between theory and practice.
Example 1. A treatment system for lagoon
sludge in Siberia was installed in several containers to be
placed onsite outdoors (Fig. 4). The core piece is a
three-phase decanter for separation of oil, water and solids in
one device. The oil is fed back into the refinerys crude stock, while
the water phase receives further treatment in the
refinerys effluent treatment plant.
Fig. 4. Containerized
treatment system with
three-phase decanter getting prepared
Due to the harsh climate and the limited choice of process
chemicals (polymeric flocculant and demulsifier), special care
has been taken to provide sufficient flexibility and hydraulic
retention time (external tanks, built by the customer) for the
chemicals to react properly, and to provide sufficient space
inside the containers to serve as dry and heated storage, as
well as a workplace for maintenance and repairs. The product
is heated in several steps using indirect steam heating.
Explosion protection was designed in accordance with
potentially explosive atmospheres (ATEX) Directive 94/9/EC.
Explosion protection required that all high voltage electrical
equipment be installed onsite in a separate container placed
outside the hazardous area. The electrical container also
included basic laboratory facilities in order to allow the
plant operators to assess the properties of the feed and
treated products and carry out adjustments as required.
Example 2. A skid-mounted treatment system
for oilfield waste in China was placed indoors and connected to
an extensive mechanical pretreatment system delivered
separately (Fig. 5).
Fig. 5. Skid-mounted
treatment system with
two-phase decanter plus three-phase
separator before shipment to China.
This plant had to be designed for a feed product with high sand
content, and for the production of high quality oil (sold to a
refinery nearby) and water (reused
internally as process water). Solid phase quality requirements
were also rather strict, as they were to be fed into a soil
In view of these requirements, the separation process was
split into two steps. The first step featured a two-phase
decanter centrifuge with the sole task of removing solids. The
next step utilized a three-phase high speed disc stack
separator to separate and clean the oil and water phases.
Similar to the decanter centrifuge, a disc stack separator also
uses differences in specific gravity for separation. However,
it provides even higher centrifugal acceleration and a higher
settling area, with the 1xg equivalent being a
As the treatment facility is situated in a remote location,
no steam was readily available and a thermal oil heating system
was used instead to provide heat to all parts of the plant.
Similar to Example 1, explosion protection is based on ATEX,
and all high voltage equipment is installed outside the
hazardous area, in the central electrical room of the
Turning challenge into opportunity.
The large quantities of oily waste produced by the oil
industry worldwide are a significant economical burden. In
order to treat this waste in the most economical way, a
detailed study of the product and of the external factors
surrounding the project is required. Advanced treatment methods
are centered around decanter centrifuges for liquid-solid
separation. These machines have proven to be the most reliable
and flexible to separate these complex oil-water-solids
mixtures under the harsh conditions typically met onsite.
Decanter centrifuges and the treatment plants in which they
are operating should be designed to the specific requirements
of each project in order to make the best
use of the versatility of the decanter and thus achieve optimal
performance. By following this groundwork, a refiner can turn a
challenge into an opportunity. HP
Hertle is the manager of sales for Hiller GmbH
in Vilsbiburg, Germany. He is a chemical engineer with a
special focus on treatment processes for liquid wastes
and slurries. In his 20 plus year career, he has worked
for a broad spectrum of companies, including those
involved in applied sciences, consulting, engineering and