The Designof targe Diameter Skim Tanks Using
Computational Fluid Dynamics (CF'D) For Maximum Oil Removal
15frAnnualProduced
WaterSeminm,
January
2005
Hilton NASA ClearLake,Houston,Texas72058
Chang-MingLee, Ph.D. andTed Frankiewicz,ph.D. -NATCO Group Inc.
2950North Loop West,Suite750,Houston,TX77O92
Abstract
A new designof internals of a large diameterskim tank is discussedin this paper.
Both transient and steady-state CFD simulations were perfomred to assist the
development of the design. Excellent flow distribution was achieved on the inlet
diskibutor as the variancein massflow ratesout of the lateralsis lessthan + 0.5%. The
retentiontime of producedwater was improved to almost 88.5% of the theoreticalvalue
with installation of two largeperforatedplates. Flow path lines were also improvedwith
a much reducedrecirculation pattern. This compareswith a residencetrme of 43.4Toof
the theoreticalvalue for the casewithout the two lmge perforatedplates. The CFD model
predictsthat oil droplets size larger than 100 microns are expectedto be fully skimmed
off without assistancefrom gas flotation for producedwater at the design flow rate of
100,000
BBL/Day.
Introduction
The production of oil and gas is usually accompaniedby the extraction of
assooiatedwater. After the initial bulk separationof the bulk produced fluids, the
produced water still contains finely dispersedsolids and oil. In order to reduce the
oontaminantcontentof dispersedoil in producedwater, a large diameterskim tank canbe
used. Sepmationis basedon the differencebetweenthe specific gravity of oil and water
andthe coalescenoe
of small oil droplets. When the retentiontimo is sufficient, oil floats
to the surfaceand can be separatedby an overflow. This techniqueis only suitable for
removingdispersedoil with a sufficiently large particle size. Dissolvedmaterialssuchas
benzeneandhealy metalscannotbe separatedusing this techaique.
The largo diameter skim tank or its modified version, parallel plate interceptor
(PPI) or comrgatedplate interceptor(CPf, is mostly usedas part of a set of techniques
for the rerroval of dispersedoil. ln this paper, we discussthe developmentof a new
configurationof intemals for a large diameterskim tank. The designof the intemalswas
developedwith aid from a seriesof CFD (ComputationalFluid Dynamics) simulations.
The objective of the CFD developeddesignis to provide a large diameterskim tank with
optimized flow pattems that maximize liquid residencetime and improve oil/water
seoaration,
Tank Description
The skim tank is desiped for a flow rate of 100,000BWPD, has a diameterof 80
feet (24.384m), and a height of 24 feet Q .3152 m). The drawing of the skim tank and
the inlet distributor me shownin Figure 1. Tho inlet distributor oonsistsofa headorand
eight parallel laterals. Becausegasbreakoutin the oiVwatersettling zonecould seriously
degrade separation performance, the fluid inlet incorporates a tangential enrry*
configuration to promote gas/liquid separation prior to liquid reaching the inlet
distributor. The open-endof eachlateral is directedtowmd the wall of the skim tank and
employs the tank wall as the equivalent of momentum breaker and primary flow
doflootor.
The skim tank has fwo large perforatedplates. One is located near the inlet
distributor and another is located between the three water outlet nozzles and the oil
skimming collectors. The large plates are an assemblyof several smaller pieces of
perforatedsheetswith thicknessof l/2 nch. The two oil skimming collectorsaf,eset at
different eievationsand they are suitablefor operatingthe skim tank at the high and low
levels of 22.5 feet and 17 feet, respectively. There are alsothreebaffles surroundingthe
outlet nozzles to prevent possible flow short circuiting. Among the three baffles, one
solid plate is positionedpmallel to the lmge perforatedplate andthe other two perforated
baffIesareboth perpendicularto the solid baffle.
Model Simplification
The constructionof the three-dimensionalgeometrywas basedon the dimensions
of the skim tank as provided by the field operatorand somesimplification was required
for modeling pulpose. The three-dimensionalmodel with a description of intemal
componentsof the skim tank is shownin Figure 2. Tlrc perforatedplatesaremodeledas
"porous-jump" boundariesof zero-thickness- the mathematicalequivalent of physical
fritted plates - in order to avoid extremely complex meshing of the region around
individual orifices. The oil skimming troughs are simplified without the V-notch slot
detailsandtheir inclination. In addition,all of the intemal piping is ignored.
During the fust stageof the project, the bulk part of the tank volume positioned
betweenthe two large perforatedplates was excluded. As a result, more details and
tighter meshingcould be appliedin the inlet region. This perrnittedthe study of the fluid
flow distribution through the inlet devioewith the computingtime still constrainedto a
manageablescalefor rururing dynamic simulations. In the later stageof the project, the
bulk volume of the tank is taken into accountbut inlet boundariesare further simplified
andassumeequalflow rate out of eachlateral in order to studythe flow patteminsidethe
skim tank. The simplified tank model is shown in Figuro 3 and meshing details of the
inlet region areshownin Figure 4.
Two LargePerforatedBafnes
Figxe2.
3-D Model of the entire skim tank with description of intemal components.
t_
Figure 3. Simplified skim tank model wio the bulk volume between the large perforated plates.
4
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Meshine of the Flow Domain
For CFD, the computationalapproachdiffers from analytical (or theoretical)
solutionsin that the CFD softwmeonly solvesequationsat specificpoints ratherthan for
the entire flow field. Properly choosing these points may become quite diffioult especially for a very complex geometry and sometimesit may require hundreds of
thousandsor millions of points. Before thesespecific points can be generated,the user
must first determine the size of the region to be modeled -- this is the so-callec
"computational domain".
The computational meshing process breaks up the
computationaldomain in space,then the calculations(solving the firndamontalequations
for mass, momentum and energy balance) are carriod out at each node point
simultaneouslyor sequentially,dependingon the selectedsolver.
The quality of the gdd is extremely important and can strongly influence the
solution- sometimesalso detemriningwhetheror not a valid or a convergedsolution can
be aohievedat all. The most important qualifioation about a computationalmeshingis
that it must define enoughpoints to captureeverythingof interestthat is happeningin the
computationaldomain without becoming so extensive that umeasonablecomputation
times are requhed. If there are too few points included, someof the critical infomration
aboutthe flow regimemay be completelylost.
The types of meshingthat arrangethe spatial points in a roughly rectangular(or
hexahedral)affangementme referred to as "structured" meshing. Another method of
meshgeneration,which involves a triangular (or tetrahedral)layout of the computational
points, is called "unstructured"meshing. Sometimes,different meshescanbe adaptedin
fifferent regions decomposedfrom the whole flow field to take advantageof specific
characteristicsof the different meshitrg systems. Such a combination is so-ca1led
'.hybrid" meshing(also including prism/wedgeandpyramid elements).
In this study, a hybrid meshingwas constructedin the first stageof the study for
the simplified skim tar* model without the bulk volume betweenthe large perforated
plates. The hybrid meshingwas comprisodof 1,519,160hybrid cells,andthe total node
countwas367,855with 3,194,845faces.As describedpreviouslyandshownin Figure4,
the surfacemeshingof the inlet headerandthe lateralsillushatesthe detailsof the tighter
meshingsoherneimplementednear the region of the inlet distributots. In the later stage
of the study, a fully structuredmeshingwas createdincluding the bulk volume of the tank
betweenthe two large perforatedplates and there are a total of 695,620hexahedralcells
with 730,041nodesand2,122,171faces.The reductionof total cell countwas dueto the
relativoly loosemeshingschemeand a firther simplification of the inlot boundmies.
Material Properties
The CFD sotwme used in this study is capable of simulating multiple phase
flows of a computationaldomain with both structuredor hybrid meshing. The fluids
used in this study were producedwater and its associatedoi1 content of7l0 ms/liter.
Underthe operatingconditions,35'c and 101.3kpa, the densityof the producedw-ateris
I I 04.3kg/m' andits viscosityis 0.7225cp,-whereas,
the densityand viscosityof rheoil
phaseat operatingconditionsare 814.9kg/m3and 3.54 cp, respectively.
Simulation Models
In this study for skim tank design, two diflerent CFD simulation models were
employed.The following is the brief descriptionof the two simulationmodels.
VOF (Volumo of Fluid) Model: It is a multiphasemodel accqrting simultaneous
different fluid flows. However,the phasesme not interpenetrating.The volume fraction
of eachphasewithin a computationalcell is determinedand usedto volume-averagethe
propertiesof the individual fluids. No particle sizes can be specified for the incoming
phasesand it is usually done with transient calculationswith small time steps. This
model was used in the first stage study focusing on the flow rate through the inlet
distribution device.
Steady-State(Water PhaseOnly) Model: The Single-PhaseSteady-Statemodel is
usedto developthe flow regime below the water surface. Therewas a virtual barrier at
the surface in order to generaterealistic flow profiles for a totally water-filled tank
model. Oncethe flow regimehas beencalculatedin the steady-statemodel, particlescan
be injected at ths inlet (or any other location of the operator'schoice) to visualize their
movementwifhin ths flow field. The propertiesof the particles (size, density, etc.) can
be chosento simulateany of the phasesbeing studied. Using this technique,it is possible
to deteffrine partiole sizes likely to be carried out with the oppositephaseas well as
retentiontimes of eachphasewithin the skim tank.
Results and Discussions
Flow Ratesof Inlet Distributor:
The mass flow rates in the inlet distributor through each lateral at different
simulationtime from 100 to 120 secondsare listed in Table 1 and the correspondingplot
is shown in Figure 5. The aboveflow rateswere calculatedduring the first stagestudy
using the VOF Model with transient simulation and a total simulation time of 120
seconds.Note that the variancein massflow ratesout of the lateralsis lessthan + 0.5%.
This constitutesexcellentflow distribution from the inlet distributor.
Table1: MassFlowRatesthroughInlet DistributorLaterals
Time
Time
'110sec
FlowRatio
Time
00 sec
FlowRatio
oA
lo
120sec
FlowRatio
yo
%
25.234
't2.55%
25A41
12.550
25.390
12.54%
Arm-82
24.401
12.14%
24.72'1
24.755
12.24o/o
Arm-83
24.684
12.28%
24"973
12.20o/o
'12.32%
25,019
12"36V"
Arm-84
26.301
13.08%
26.912
12.98%
26.141
12.s1%
Arm-F1
25.280
12.58%
25.490
12.58o/o
25.442
12.5tvo
Arm-F2
24.g16
12.10Yo
24.632
12.15%
24.690
12.20%
Arm-F3
24.525
12.20%
24.8'14
12.24%
24.860
12.280
Nm+4
26.278
1i.O7%
26.291
12.97%
26.122
12.90%
Summation
2O1.O19 100.00e/o 202.674
100"00%
202.448
100.00%
lfl
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a-u
t
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Ete
l4
IU
Sloulation Tlne {sec}
Figure 5. Massflow ratesout of ind:ividualinlet distributorlaterals.
Skim Tank Retention Timet
oil and water separationis fundamentallya graity separationprocess. Almost
any production facility can gain significant benefits &om inclusion of a gravity separator
in the primary stage of producedwater treaknent. However, the primary limitation of
successlul gravity separation is the size of the vessel required to prwide adequate
retsntion timo for fi;ll heatrnentand the efficient use of availabletank volume. In tlFD
simulations, produced water retention times were detennined by injecting water
"particles" into the model and tracking their time to exit the outlet nozzles. Thi results
are shown in Figure 6 and Figure 7, in histogram plots and cumulative curves as a
percentageof total particles exiting, for skim tank intemals without the two larse
perforatedplatesand after the installation of the two lmge perforatedplates.
A comparison of the theoretioal (volumetric displacement)retention times for
produced water with the median retention times determinedby cFD analysis also is
shown in the table on the right-hand-sidein both Figures 6 and 7. The theoretical
retentiontime of the skim tank is 17,406secondsbasedon a liquid level of 225 feet witl
a total producedwater flow rate of 100,000BBL/Day. As can be seenfrom the tables,
water retentiontime for the skim tank without the two perforatedplateswas abofi 43.4yo
of the theoreticalvalue. This is a clear indication that short circuiting occursinside the
skim tank. On the other hand, tle retention time of producedwater was improved to
almost88.5% of the theoreticalvalue after installation of the two large perforatedplates.
Furthermore,minimum value of the retention time for the latter was 7434 secondsas
comparedto 634 secondsfor the former case. These results suggesteda significant
improvementof effectively utilizing the bulk part of the tank volume when implementing
the designand installation of the two largeperforatedplates.
Fluid Flow Patterns:
As traced by the flow pathJines plots of the CFD software,the fluid flow paths,
coloredby retentiontime, inside the skim tank without and with the two large perforated
plates are shown in Figures 8(a) and 8(b), respectively. In Figure 8(a), ttrere me lmge
swirling pattemsthat are characteristicsof severerecirculationand the presenceof shortcircuiting flow paths inside the skim tank. In contrast,the path lines are improved in
Figure 8(b) with a much reduced recirculation pattern. In separatesimulations, the
calculationsshowed similar flow patterns with a produced water flow rate of 30,000
BBL/Day and the skim tank liquid level set at l7 feet insteadof the oiginal 22.5 feat.
The plots of flow path lines for thoselow flow simulationsareillustrated in Figures9 (a)
and9(b).
Carry-under of OiI dropletsin Water Phase:
In order to evaluatethe sepmationperformanceof the skim tank with desigaed
internals,the entrainmentof oil dropletsin the water phasewas studiedby the injection
of various sizes of oil "particles" into the water phaseat the jnlet and then determ:ining
their potential for carry-underby following the particle hacks generatedby the CFD
simulations. It should be noted that current cFD modeling is unablo to simulato the
coalescenceof oil droplets over time. Therefore,the sepmationperfomranceillustrated
hereis for the worst casescenariowherethe reverseemulsionis fully stabilized.
The CFD model used in thesedeterminationswas the single-phasesteady-state
model with a virtual barrier placedat the surf,aceas previously described. The sideview
of the tank with particle tracking for 50 Fm, 75 pm, and 100 pm oil dropletsareshownin
Figures 10' 1l' and 12, respectively. Examination of these figures showsthat there is
substantialenhainrnentofthe 50 pm oil droplets,only slight entuainmentofthe 75 pn oil
droplets, and no entrainment of the 100 pm droplets. In summary, with the
implernentationof designedinternals,the oil dropletsof size larger than 100microns are
expectedto be readily slcimmedoff without exha assistancefrom gasflotation inside the
skim tank at a producedwater flow rate of 100,000BBL/Day. In a later study with a
lower producedwater flow rate of 30,000BBL/Day and skim tank liquid lovel set at 17
feet, the minimum oil droplet sizethat was readily skimmedoffwas reducedto about50
microns.
Conclusions
Computationalfluid dynamics simulations were perfomred on a lmge diameter
skim tank and the results indicate that a satisfactoryperformanceof the tank can be
achievedwith the proposedintemal configuration under the specified producedwater
flow rate provided that the incoming oil droplets are not too small and that bulk gas
evolution in the inlet region is eliminated or minimized. With the installation of two
large perforatedplates,the "Median" value of the estimatedretentiontime from particle
traoking is 88.5% of the calculatedtheoretical retention timo. This suggestsa very
effectiveutilization of the bulk part ofthe skim tank volume.
The results from transient CFD simulations demonstratethat an excellent flow
distribution throughthe inlet distributor is achievedwith the variancein massflow rates
out of eachlateral being less than + 0.5%. From the particle tracking results,the CFD
model predictsthat oil dropletslargerthan 100microns are expectedto be fully skimmed
off without assistancefrom gas flotation at a produoed water flow rate of 100,000
BBL/Day.
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