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Selection GuiaeT
Centrifugal
Oil and Water
beparations
orce powers lar e-scale
water separations.
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By Robert C. Waterbury, Senior Editor
any industrial process- i than60,OOOtimes that ofnormal gravity i Rotation forces the heavier liquid to the
In rotating the tank orbowl to accelbowl periphery while the lighter liquid
es entail separating one i
remains in the center, closer to the axis.
liquid fromanother. In ; erate separation of the two liquids, the
the food, pharmaceuti- i higher the speed the greater the cen- ;
Two primary devices, centrifuges
cal and fuel oil industrifugal force acting to effectseparation. i and hydrocyclones, are used in largetries, for example,there ; Industrial centrifugesmayuse holes cut i scale industrial separations. We will
is frequently a need for i into conical diaphragms or discs to feed i look at where and how each is used in the
the mixture. The mixture then passes i various process industries.
oil and water separa-
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beTwo
separated
immiscible
because
liquids
one islike
lighter
this
into
the holes
the bowl
to the
axis
separation
and flowschamber.
through i
can
tions.
than the other. Thus, using baffles or
diaphragms, it is possible to separate
them in a tank or vessel. Historically,
these separations have been performed
using filtration or gravity processes.
According to Stokes' Law, a particle
moving through a viscous liquid attains a
steadyvelocityor sedimentationrate.But
this rate can be quite low in normalgravity if the viscosity of the liquidishigh, ifthe
particle diameters are quite small,or if the
particle densitiesare similar.
Centrifugal force is often used in
components.
industry to accelerate the natural sepai
ration process-sometimesatforcesmore i
CENTRIFUGES
Centrifugescan be used to separate
liquids from solids, two different liquids, or a combination of several liquids and a solid. There are four major
types of centrifuges:disc-stack,decanter,
tubular bowl and solid bowl basket.
Within the disc-stackcategory,various '
designs are further distinguished by
how they handle solids:
The solids-retaining type requires
periodic manual removal of solids.
mThe solids-ejecting type automatcally removes solidsat predetermined
intervals.
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cn used for liquid-liquid separa-
the tubular bowl and the disc-
effectivenessby shortening the
withverysimilarspecific gravities.For
this reason, they are often used to recover high value product or make difficult
separations in the chemical and pharmaceutical industries. Tubular bowl
applications typicallyinvolve low solids
content in the feed stream and low
throughput requirements.
InformationsubmittedbyCentrico,
Inc. for Westfalia Separator AG indicates that typical centrifugal separator
applicationsinvolveseparatingsolidsin
the0.2-10pmkangewithminimumdensity differences of 30-300 kg/m3, and
separating liquid mixtures with small
density differences in the range of 20400kg/m3.
Water monitoring is an important
consideration when the density differences between oil and separable water
are small. A water monitoring system
developed by Westfalia continuously
monitors water content directlv in the
bowl instead of in the clean oil &charge.
The system consists of a water sensor
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sedwidelyin clarification
centrifuges the process fluid is
the centrifuge provides a large
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yz:z :-$i'zo:
tzzmge
feed
feed product passes
stack, solids and/or
Periodic shutdown and disassembly
are required for cleaning. An alarm
equipped with a flow switch is providedin a separate water line to detect seal
water loss. Disc-stackcentrifuges with
self-cleaning bowls operate much the
same way except the bowls can evacuate the solids automatically.This eliminates the need to stop the machine for
Tubular bowl centrifuges (Figure
2) operate at very high speed and thus
generate the greatest spin and the highest G forces (from 13,000to more than
60,000G).This capabilityenables tubular bowl centrifuges to clarify liquids
and separate two immiscible liquids
8 FiltrationSystems Mar/Apr 1996
Figure 2. Typical bowl construction o
f an oil sepamtor
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(conductivitysensor) and two solenoid i characteristic of hydrocyclones. They
valves, the water discharge valve and i canbeusedsinglyorincombinationin
the circuit valve.
i a manifold or multicone arrangementto
accommodate a wide range of required
HYDROCYCLONES
i flowrates while providing high separaHydrocyclones also use centrifu- i tion efficiency.With no moving parts,
gal force to achieve separation, classifi- i they are highly tolerant of changes in
cation and dewatering in process i flow, motionand concentration.
Because they are relatively unafapplications (Figure 3). Hydrocyclones i
use centrifugal force created by the i fected by temperature, pressure, abraincomingfeedstream to separate the liq- i sion and other environmental factors,
uids; there are no moving parts. In oil i hydrocyclonesareusedinawidevariety
and water separations the force causes i ofapplications andindustries-minerthe oil to flowaxially up the center of the ! a1 processing, petroleum production
device and out the top. The water is i andrefining, coal processing andmany
forced down the inner wall of the hydro- i wastewater and chemical processes.
cyclone and out the tailpipe.
i
In petroleum processing, for examHydrocyclones are used to con- i ple, hydrocyclones are used to remove
centrateslurries, separate liquids,degrit i oil and solids fromproducedwater and
liquids, and wash and classify solids in i petrochemical wastewaters. They help
1iquidsuspensjons.Theyarealsousedas i remove sand and grit from all types of
protection or pretreatment devices to i crude oil, drilling mud, oiYwater mixreduce costs or improve performance i tures andrefinedproducts, and they are
of equipment such as pumps, valves, i instrumental in catalyst recovery and
centrifuges, filters and screens. They i removal of coke solids from oil during
are used alone or in combination with i the refining processes. *
thickeners,clarifiers,filters andscreens. j
Hydrocycloneshave proven effecHigh throughput is an important i tive in removing oil and solids from all
i types ofindustrial intake water, process
i water, wastewater and coolants.Oil and
i solids removal not only clarifies the liqi uid but protects process equipmentfrom
i abrasivewear.
i
The modular design of hydrocyi clone devices permits selection of varii ous inlets, cones and tailpipes to
i optimize capacity,oil removal efficieni cy and pressure drop for a number of
i applications.
MARINE OIL S P I L L
CENTRIFUGE
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Xecovery of spilled oii and hydrocarbon fuels in marine waters is a complex, ongoing challenge.Weather, time
and the constantly changing conditions
ofspilledcrudeoils,tarsandasphaltscan
i make recoveryoperationsboth difficult
i and ineffective over time.
. A simple, portable liquid-liquid
i centrifuge based on technology transferred from the Department of Energy
and further developed over 18months
at CINC, Inc., promises greatly
i improvedeffectiveness.Ituseson1yone
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Figure 3. Separated feed llquid is
dischawed as overfiow in a hyuroCyclone. Solids move down the
andgoorrtoftheapex
underflow.
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i moving part and includes an annulus
i in the area between the rotor and the
i rotor housing.
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Two liquidphases, either premixed
i or separate, are pumped into the annu-
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Ius between the housing and the rotor.
Thespinningrotormixes the twophases to form a uniform dispersion. The
dispersed mixture then flows downward to the bottom plate where it is
duected toward the center of therotor by
radial vanes. The dispersion is rapidly
accelerated to rotor speed as it enters
therotor.Baffleswithintherotorprevent
remixing andallow separation to occur
under a force of 200-300G. The denser
phase is pushed to the periphery of the
rotor and displaces the lighter phase
which moves toward the rotor center.
Weirs control the rate of escape for each
phase and are used to set the desired
interface position between the phases.
The separate phases leave the rotor and
are collectedin tworegonsoftheupper
housing leading to exitports.
The rotor functions as a pump and
fills when it is spinning. This is important because it allowsequilibrium to be
maintainedduring changesof ratios and
flow, or in case of total flow interruption. Startup equilibrium is achieved in
30 seconds.Large changesin phase ratio
are tolerated due to the dispersion
formed in the mixing annulus. Air
pumped from skimmer operationsdoes
not affectequilibriumbecauseitbypasses the rotor and exits through the liquid
outlets. The centrifuge'sefficientdesign
and the forces it generates allow it to
operate at a tilt angle of 3 5 O with no
adverse effect on performance.
Tests were conducted using No. 2
diesel fuel to determine separation
behavior in simulated fuel and solvent
spill cleanup. IS0 460 gear oilwas used
to study heavier oil separation from
water. Finally, two different types of
crude, Alaska North Slope (ANS) and
SanJoaquin (SJ),were analyzedthrough
an outside contract laboratory. Three
separator sizes used in the tests covered
2-6 gpm,
and 80-200 gpm
-~ 10-30gpm
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operifting ranges. To date, separation
efficienciesofmorethan99%havebeen
achieved over a wide range of oil vis-
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MarIApr 1996 Separation a FlltratlonSystems /I,
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particles in the submicron range. Centrifuges are used in separating heavy
oils and highlyviscous substancessuch
as dehydrating oils.
But high performance comes with
a high price tag. According to Hank
Rawlins of Krebs Engineers, hydrocyclones capable of performing simple
separations down to 5 microns at flows
of 5-10 gpm cost less than $5,000. The
cost of a centrifuge to do the same job
would probably r u n i n excess of
$100,000.
"Furthermore,hydrocyclones may
pull about 2000Gswhereas centrifuges
go up to 20,000Gs or more,'' says Rawlins. "Hydrocyclonesaremeantforcoarse
or rough separations such as removing
the bulk quantity of oil down to maybe
20 or 30ppm. Centrifuges,on the other
hand, do fine polishing separations
down to less than 1 ppm of oil. For that
reason you might find a hydrocyclone
performingrough separationsupstream
i of a centrifuge or pumps and other
i process equipment. Hydrocyclonesare
i also highly resistant to ?brasives and
i offer greater throughput per size. And
i multiple units can bepackagedinmani ifold or multicore arrangements to proi vide virtually unlimited flow rates. But
i they arenot designed to do the extremely fine separations performed by ceni trifuges."
Althoughcentrifugalforcepowers
i oil and water separations in both centrifuges and hydrocyclones, the two
i devicesrarely competein specificapplii cations. Clearly,the separationrequirei ments of the application will point to
use of one or the other, but hardly ever
i both. The difficulty of separation, feed
i rate, type of separation duty or system
i performance, and the size and conceni tration of solids particles, if any, will
i largely determine the choice between
i these devices. Q
i
:
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1
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" ~ w " p & p b
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CIRCLE READER SERVICE NO. 219
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