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Oil/water separation technologies
Emulsions can cause fouling andunder-deposit corrosion prob-lems
in distillation columns,
heat exchangers and reboilers.Commercial methods for
breakingemulsions include settling, heating,
distillation, centrifuging, electricaltreatment, chemical
treatment andltration. These separation technol-ogies can be used
in combination tosecure optimum results.
Emulsions can be classied asoil-in-water and water-in-oil
types.The type of emulsion can be deter-mined by adding a drop
ofemulsion into a beaker containingwater and oil. If the emulsion
is ofthe water-in-oil type, the drop
diffuses through the oil but remainsin water. The oil-in-water
emulsiondiffuses through the water, but notthrough the oil. Both
types of emul-sions can co-exist in crude oil side
by side.
Separation of water-in-oil emulsionsIn this type of emulsion,
water isthe internal dispersed or discontin-uous phase, while oil
is the externalor continuous phase. Separation bythe different
gravity of the two
phases is a very slow process, butcan be accelerated by the
assistanceof chemicals. The chemicals usedare termed demulsiers,
emulsion
breakers or wetting agents. Theseadditives are
surfactants, whichmigrate to the oil/water interface.They adsorb on
the oil lmssurrounding water droplets and
break the oil lms. Then, waterdroplets aggregate to form
waterdrops large enough to gravitation-
ally separate them from the oil.Non-ionic surfactants having
bothlipophilic and hydrophilic
Where stable emulsions cannot be removed mechanically, the
application of
demulsifiers, coagulants and flocculants accelerates the
separation process
BERTHOLD OTZISK
Kurita Europe
groups are mainly used asdemulsiers.
Typical applications in refineriesTank farm treatmentCrude oil,
intermediates and
nished products are stored in thetank farm. It is the rst
facility in arenery where free water can beremoved by settling from
the oil.Pumped crude oil from the wellcontains water in emulsied
andfree states. A crude oil emulsionconsists of small globules of
watersurrounded by oil. Water is theinternal phase and oil is the
exter-
nal phase, which can easily bedetected by microscope. With
thehelp of gravity, small water drop-lets coalesce to form
biggerdroplets. An adequate residencetime is essential for
separation intotwo phases. The bigger dropletsnally settle down to
be removed
by drainage.Most of the time, emulsied
water cannot be separated effec-tively by gravity settling only,
as
the emulsion can separate intothree phases:• Oil on the top
• Water at the bottom• Persistent emulsion in themiddle or below
the water layer.
To break such a persistent emul-sion, chemicals have to be
applied.A number of demulsiers are
commercially available with vary-ing degrees of performance
andselectivity. Generally, demulsiersare diluted with an organic
solventand injected into crude oils. Thenature of the emulsion
changesfrom crude to crude, which caninuence the performance of
theemulsion breaker programme. Thisnecessitates the evaluation of
costeffectiveness and performance in
breaking the emulsion.
Crude oil desaltingCrude oil fed from the tank farm tothe crude
distillation unit stillcontains water, salts, sludge andvarious
kinds of impurities. Thiscan cause corrosion, fouling, plug-ging
and catalyst degradation inthe downstream rening units. Themain
purpose of electrostaticdesalting is therefore to removeimpurities,
such as inorganicmicroparticles, suspended solids
and water-soluble contaminants,together with the water.
The major variables and effectson the desalter operation are:•
Wash water mixing• Wash water quality and rate• Desalting
temperature• Electric eld• Retention time• Use of demulsiers.
Wash water is added in front ofthe mixing valve to the crude oil
to
prepare a temporary emulsion. Akey point of desalting is an
appro-priate mixing of crude oil with the
www.eptq.com PTQ Q2 2013 87
The nature of the
emulsion changes
from crude to crude,
which can influence
the performance of
the emulsion breaker
programme
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88 PTQ Q2 2013 www.eptq.com
forces. A cationiccharged long-chain poly-mer neutralises
thenegatively charged oildroplets. The repellentforces are weakened
andoil droplets are broughttogether. This resolvesthe emulsion of
waterand oil. The emul-sion-breaking processinvolves three steps:•
Agglomeration• Creaming
• Coalescence.Figure 2 illustrates the physical
process of oil-in-water emulsion breaking. Agglomeration is
theassociation of small dispersedphase droplets (clusters).
Creamingis the concentration of the dispersed
phase. Coalescence is the drainageof the continuous phase. The
oildroplets agglomerate by forming
bigger droplets and clusters, andare collected at the
surface. Theaddition of an emulsion breakeradditive helps to
accelerate theseparation process. These types ofemulsion breakers
are surfaceactive components, which destabi-lise the dispersed
phase.
Typical applications in refineries andpetrochemical
plantsEthylene productionEthylene is mainly produced bysteam
cracking. This processincludes thermal cracking,
cooling,compression and separation. Lightliquid hydrocarbons
(naphtha) andgases are converted mainly intounsaturated smaller
molecules,which are separated by compres-sion and distillation. The
hot gasesleaving the cracking furnaces are
immediately quenched in oilquench and water quench columns.The
purpose of the cooling is toprevent polymerisation and theformation
of unwanted byproducts.The collected quench water is sepa-rated
from heavy hydrocarbons inthe oil/water separator. Often,
theseparated quench water stillcontains hydrocarbons, which
aredispersed in the aqueous phase.
Demulsiers are usually applied
to improve the separation of hydro-carbons from the quench
water. Itis mandatory to provide the correct
wash water to obtain asufcient desalting rate.Heating lowers the
viscos-ity of crude oil. Thispromotes demulsicationand the
formation of largewater droplets from theemulsion. An electric
eldis induced by AC or DCcurrent in the oil/watermixture to improve
watercoalescence. The electricaleld imposes an electricalcharge on
the small waterdroplets entrained in the temporaryemulsion. The
water dropletscoalesce into bigger droplets, whichcan settle by
gravity. Therefore,sufcient retention time in adesalter is required
for efcientwater and oil separation. A suitable
demulsier is commonly used topromote the separation of waterand
oil. The desalted crude oil iscontinuously fed from the
desaltervessel to the atmospheric crudedistillation column. The
desalterefuent water is discharged fromthe desalter vessel to the
wastewa-ter treatment facility.
Figure 1 shows the laboratoryevaluation of demulsiers
incomparison with an untreated
crude oil sample (blank). For theevaluation of a demulsier,
thecrude oil was mixed with 4 wt%wash water and agitated with
anelectric stirrer. This mixture wastransferred into several
centrifuge
glasses. With the exception of the blank sample, 10 ppm of
differentdemulsiers were added into thecentrifuge glasses and
thoroughlymixed again. After 24 hours, thewater content and salt
content wasdetermined to nd the best
performing demulsier. In thiscase, Kurita EB-4110 and
KuritaEB-4113 showed the highest desalt-ing and dehydration
efciency forthis crude oil. EB-4110 is an oil-soluble demulsier,
which is typi-cally injected into crude oil in frontof the desalter
mixing valve.EB-4113 is a water-soluble demulsi-er, which is
typically injected intodesalter wash water.
Separation of oil-in-water emulsionsIn aqueous systems, the
hydrocar- bons generally carry a negativecharge at their
surface. Often, theyare steady dispersed into smalldroplets because
of their repellent
Figure 1 Evaluation of demulsifiers
Emulsion
Agglomeration
Coalescence
Creaming
Creaming and
coalescence
Figure 2 Oil-in-water emulsion breaking process
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amount of chemical to avoidoverdosing of the demulsier,
because at higher concentrationsdemulsiers have the
tendency toact as emulsiers — instead of
breaking the emulsion, they form amore stable emulsion
with hydro-carbons. A simple beaker test is agood tool to check how
muchdemulsier is required for the sepa-ration of hydrocarbons and
water.
Figure 3 shows the result of alaboratory evaluation to nd
the
best performing emulsion breakerfor quench water. In this
case, nodemulsier was applied in front ofthe oil/water separator.
The sepa-rated water still containedhydrocarbons, which were
dispersedin the aqueous phase. It was notpossible to remove these
hydrocar-
bons in the oil/water separator. Thetarget of the
laboratory test was toselect an emulsion breaker thatsignicantly
improves the removalof hydrocarbons from water within10 minutes’
residence time. In thiscase, the emulsion breaker added to
bottle No. 3 showed the best perfor-mance. Within the
dened timeframe, an impressive hydrocarbonlayer was formed, while
othersamples showed no effects or poor
separation. The good results of thelaboratory test were conrmed
laterin a eld trial.
Wastewater treatmentWater is used intensively in rener-ies and
petrochemical processes,and during its use it becomescontaminated
with hydrocarbons,increasing the biological (BOD) andchemical
oxygen demand (COD) ofthe efuent water. Cooling water,process
efuents, rain water and
surface water are collected at thewastewater plant together with
avery briny efuent stream from thedesalting process. Typical
pollut-ants are hydrogen sulphide,ammonia, cyanides, metals
andsuspended solids. Effective wastetreatment technologies are
requiredto comply with all legal require-ments. The wastewater
treatmentmethods are generally classiedinto three categories of
mechanical,
chemical and biological treatments,and a wastewater plant is
typicallydesigned in three steps:
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• Mechanical separation• Chemical and biological treatment•
Filtration and polishing.
The main purpose of the primarytreatment is the separation of
oiland water. The removal of oil is avery important step to reduce
theloading to the downstream treat-ment processes, because high
oillevels are toxic for the microorgan-isms used in biological
treatment.The wastewater typically containsoil in water emulsions,
with oildispersed in the continuous waterphase.
Common primary treatment unitsare:• API oil/water separator•
Corrugated plate interceptor(CPI)• Dissolved air otation (DAF)•
Induced air otation (IAF).
The function of an API oil/waterseparator is based on the
differencein specic gravity of oil and water.Suspended heavy
particles settle tothe bottom of the separator to bescraped by a
rake into the sludge
pit, which is discharged continu-ously. The oil rises to the top
of theseparator. The wastewater accumu-lates as a middle layer
between thesettled solids and the oil phase. Itcan be sent to a
otation unit forfurther treatment. Substances suchas oil or
particles can be separatedfrom water by otation. Mechanicalotation
and dissolved air otationare applied to increase the
otatingvelocity of particles. Fine air
bubbles are generated in water. Theupward ow of the
bubbles and theadhesion of bubbles with particles
improves the efciency of the ota-tion. The oating oil is
skimmedcontinuously to be pumped into the
slop oil system.
ConclusionsOil/water emulsions appear inmany areas of reneries
and petro-chemical plants, and can causeoperational problems as
well asinfringement of environmentalregulations. There is a variety
ofseparation equipment available onthe market to separate the oil
phasefrom the water phase. In such
cases, where stable emulsions areformed that cannot be
removedmechanically, the application ofdemulsiers, coagulants and
occu-lants accelerates the separationprocess and improves
mechanicalperformance. This helps to fulllegal requirements,
reduces corro-sion and fouling risks, and resultsin reduced
maintenance costs andhigher equipment availability.
Further reading1 Kurita Handbook of Water Treatment ,
2nd
English Ed, Kurita Water Industries Ltd, Japan,
1999.
2 Ullmann´s Encyclopedia of Industrial
Chemistry – The Ultimate Reference, Release
2012, 8th Ed, Wiley Online Library, Wiley-VCH.
3 Hartinger L, Handbuch der Abwasser-und
Recyclingtechnik, 2nd Auflage, 1991, Hanser,
Germany.
Berthold Otzisk is a Consulting Engineer in the
Technical Department of Kurita Europe GmbH,
Viersen, Germany, where he focuses on refineryand petrochemical
applications.
Email: [email protected]
Figure 3 Quench water from oil/water separator
