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7/28/2019 Oil and Gas Produced Water_Treatment Technologies
1/14
Technical BriefOil and Gas Produced WaterTreatment Technologies
the
water sustainability solution
7/28/2019 Oil and Gas Produced Water_Treatment Technologies
2/14
1
PRODUCED AND HYDRAULIC FRACTURING FLOWBACK WATERS
Producedwaters,co-producedduringtheextractionofoilandnaturalgasreserves,inaddition
to theflow backwater from hydraulic fracturingactivities (i.e., fracwater)must beproperly
managedinordertomitigateanyenvironmentalimpactsandimpactstoexistingwatersupplies
by energy development activities [1]. Hydraulic fracturing is typically used to open up tightgeologicformationsorreservoirrock(e.g.,shaleformations)sothatthenaturalgasmaybe
moreeasilyextracted.Recentestimatesfortheamountofproducedwaterthatisgeneratedin
the United States (US) range from 1.6 to 2.1million gallons per day (mgd) [1]. As energy
exploration and extraction continue to increase (e.g., oil shale and coal bed methane
development) these volumesofwater willlikely continue to increase [2]. Thechemistry and
compositionofproducedandhydraulicfracturingflowbackwatersishighlyvariableandinmany
cases quite complex. The most significant concern for developing effective management
strategies for these waters is removing, or reducing, the total dissolved solids (TDS)
concentrationpriortoreuse.Thistechnicalbriefprovidesanintroductiontosomeofthemore
commonlyemployed treatment strategies forproduced/hydraulic fracturing flowbackwaters.
Emphasisisplacedonthecurrentmaturationstateofthesetechnologiesandaddressessomeof
theassociatedadvantagesanddisadvantageswiththeiruseformanagingproducedwaters.
OilandGasproducedwatersare commonlycharacterizedbyhighsaltconcentrationswhich
requirestheirdisposalinevaporationponds.
ProducedWaterChemistryandComposition. ProducedwatersProducedwatersaregenerally
characterizedasbrackishwatersolutionscontaininghighconcentrationsofdissolvedminerals,
metals, and salts [1, 3-5] (Table 1). Waters that are characterized by relatively high TDS
concentrations (> 1,000 mg/L) require some form of treatment prior to their discharge or
beneficial reuse [6-8].For comparison thesecondarydrinking water standardforTDS is 500
mg/L as established by the United States Environmental Protection Agency (USEPA).
Additionally,producedwaterscancontainhighlevelsoforganicslikeoils,greases,andbenzene,
toluene, ethylbenzene, and xylene (BTEX) compounds [1]. The specific composition and
chemistryofproducedwatersissitespecificandinfactvarydependentonthelocationandtype
of geologic formation from which the producedwater is extracted [1, 2]. Furthermore, the
chemistryandcompositionofproducedwaterfromasinglesourcemayfluctuategreatlyduring
theoperationofthewell.Thisfactrequiresthattheassociatedtreatmentsystembeflexibleso
that it can accommodate changes in the feedwater quality. Despite the variation inwater
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qualities producedwaters tend to have relatively high TDS concentrations that make them
unsuitable formostpotablewater applicationswithout treatment. Indeed,produced waters
mayhaveTDSconcentrationsthatapproach,orareinexcessof,170,000mg/L,whichisnearly
fivetimesthatofseawater(TDS~36,000mg/L).
Table1.Concentrationsofcommoninorganicandorganicconstituentsinproducedwaters
(adaptedfrom[1,2]).
Constituent Low Medium High
TDS,mg/L 1,000 32,300 400,000
Sodium,mg/L nd 9,400 150,000
Chloride,mg/L nd 29,000 250,000
Barium,mg/L nd n/a 850
Strontium,mg/L nd n/a 6,250
Sulfate,mg/L nd 500 15,000
Bicarbonate,mg/L nd 400 15,000
Calcium,mg/L nd 1,500 74,000
Totalorganiccarbon,mg/L nd n/a 1,700
Totalvolatileorganics,mg/L 0.39 n/a 35
Totalrecoverableoilandgrease,mg/L 6.90 39.8 210
ndvalueisbelowthedetectionlimitoftheanalyticalequipmentused
n/adatanotavailable
HydraulicFracturing(Fracking).Flow-BackWaterChemistryandComposition.Unlikeproduced
watersthechemistryandcompositionoffracwaterispoorlycharacterized.Thereasonforthis
isthefactthatdifferententitiesmayaddproprietarychemicalsandotheradditivesthatarenot
disclosedtothepublic.Generallyspeakinghowever,fracwaterisbrackish(TDS>10,000mg/L)
and contains various organic additives and volatile organic compounds. Example chemical
additivestofracwaterincludepotassiumchloride,guargum,ethyleneglycol,sodiumcarbonate,
potassiumcarbonate,sodiumchloride,boratesalts,citricacid,glutaraldehyde,acid,petroleum
distillate,andisopropanol[9].Frackingrequireslargequantitiesofwatertodegreeofroughly2
to5milliongallonsoffracwaterperwell[10]Notethatasinglewellmaybefrackedovera
dozentimesduringitslifetime.Approximately15%to80%oftheinjectedfracwaterreturnsto
thesurfaceasflowbackwater.
TreatmentCosts.Thecostsassociatedwithmanagingandtreatingproducedand/orfracwaters
can is highly dependent on the chemistry/composition of the raw water and the requiredfinishedwaterquality.Therefore,estimatingthecostsformanagingthesewatersiscomplexat
bestgiventhewidevariabilityinthechemistryofproduced/fracwaters.Insomecasesthecost
of treating the produced water can be prohibitive to energy development ventures.
Furthermore, as clean water is a scarce resource, treating and reusing these waters for
beneficialapplications(i.e.,forirrigation,industrialprocesses,fracwatermakeup,orothernon-
potablepurposes)mayhavesignificanteconomicincentives(ProducedWaterUtilizationActof
2008,H.R.2339).Forhydraulicfracturingrecoveringandreusingtheflowbackwatercanreduce
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costsassociatedwithdisposingofthewastewaterandtheacquisition/transportofnewmakeup
water.Essentialtotherealizationofthesebeneficialreuseapplicationsisthedevelopmentand
implementation of effective produced water treatment systems; however, the complex
chemistriesthatcharacterizethesewatersmakestreatmentbyexistingdesaltingtechnologies
difficultatbest.
RemovingTDSfromanywater isan energyintensiveendeavor.Generallyspeakingtreatment
costswillincreaseratherrapidlyastheTDSconcentrationincreases.Formembraneprocesses,
such as reverse osmosis (RO) this relationship between cost and TDS is attributed to the
relationshipbetweensaltconcentrationandosmoticpressure(i.e.,assalinityincreasessotoo
doestheosmoticpressureof thesolution).Moresalinesolutionswillrequire largerandmore
energyintensivefeedpumpsinorderto overcome theosmoticpressureof thefeedsolution.
Thetypeofdesalinationtechnologyusedwillvarydependingontheioniccompositionofthe
water.Forexample, ionexchangeorpHadjustmentmaybeusedwhenthewaterisprimarily
composed of carbonate species,whilemembrane processes or distillation processes will be
required formore complexwaters.Uniqueconsiderationsassociatedwith producedand frac
watertreatmentsystemsareoutlinedbelow:
Treatmentsystemmobilitytoaccountforthevariablelifetimesofproducingwellsaswellasthedevelopmentofnewones.
High source water recovery to mitigate the further treatment and/or disposal ofwastewatersresultingfromthetreatmentoftheproduced/fracwater.
Variability in source water quality requires that systems be flexible and robust toaccount for changes inwaterquality during thematurationofawell,as well as the
differentwaterqualitiesfromnewlydevelopedwells.
Treatment / finished water quality requirements, together with the chemistry /compositionof theproduced / fracwater,dictatethetypeoftreatmentthatwillberequired.Assuch,theleveloftreatment,andthusthecostoftreatment,mayvaryfrom
onelocationtothenext.
TREATMENTREQUIREMENTSANDCHALLENGES
Theleveloftreatmentthatisrequiredisdictatedbytheintendedapplicationorenduseforthe
treated produced water. Regardless of the intended application however, some form of
treatmentwilllikelyberequiredinordertomeettheregulatorycriteriaforthetargetedend
use.Ascleanwaterisascarceresource,treatingproducedwatermayhavesignificanteconomic
incentives,suchasitsexpandeduseasirrigationwater,processmake-upwater,orevenasa
drinkingsource(ProducedWaterUtilizationActof2008,H.R.2339).WhiletheexactchemistryandcompositionofproducedwatersisvariableitgenerallycontainshighconcentrationsofTDS
andvolatileorganiccompounds(VOCs).HydrocarbonproductsandVOCsmayberemovedusing
a range of conventional treatment systems, such as oil water separators, aeration systems,
dissolvedairflotation,andoxidation processes.Effluents fromtheseconventional treatments
usuallymeettherequirementsforsurfaceholdingpondsandsubsurfaceinjection;however,the
exceptionally high TDS concentrations in produced water present unique and substantial
challenges.The need forreducingTDS concentrations isespecially important forareaswhere
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salinity management is critical, such as for the Colorado River Basin [11]. High TDS
concentrationsarein fact,problematiceven forunderground injection,as a resultofmineral
scalingfromcalciumcarbonateandbariumsulfate,whichcanplugsubsurfaceformations.
Advancedseparationprocesses,whicharecollectivelyreferredtoasdesalinationprocessesare
required. Examples of desalination technologies include electrochemical processes, ion
exchange (IX), mechanical evaporation processes [multi-effect distillation (MED), multi-stage
flash (MSF)distillation, andvapor compression (VC)],capacitivedeionization, pressure driven
membrane processes, and non-pressure driven membrane processes (e.g., membrane
distillation, forward osmosis, electrodialysis reversal). Of these different demineralization
techniquesonly IX,mechanicalevaporationordistillation,andthemembraneprocesseshave
receivedwidespreadapplicationinthetreatmentofproducedwaters.Mechanicalevaporation
processeshavebeenusedtotreatproducedwatersfromavarietyof sourcessuchastheFort
McMurray, Alberta tar sands; however, evaporative processes suffer from a number of
drawbacks.Forexample,large-scalemechanicalevaporationsystemsareenergyintensiveand
complex.Nevertheless,evaporativeprocessesareinsomecasesthebestandonlyoptionfor
treatingchallengingwatersources(TDS>>50,000mg/L).Processeslikecapacitivedeionization
areintheearlystagesofdevelopmentandhaveyettobetestedonareasonablescale,thoughearlyresultsarepromising[7].Thefollowingsectionsareintendedtoprovideabriefoverview
ofaselectnumberoftreatmentprocessesthatarecommonlyusedintreatingproducedandto
someextentfracwaters.
PRODUCED WATER TREATMENT SYSTEMS
Pressure DrivenMembrane Processes. Pressure-drivenmembrane processes areperhaps the
mostwell known desalting technology and includeprocesses such asnanofiltration (NF) and
reverseosmosis(RO).NFisdifferentiatedfromROinthatitisprimarilyusedtoremovalmulti-
valentionslikecalciumandmagnesiumandiscommonlyreferredtoasmembranesoftening.In
addition to NF and RO are several design variations that are meant tomitigatemembrane
foulinginanattempttomaximizetheachievablefeedwaterrecoveryratio.BothNFandRO
havelongbeenusedfortreatingsalinewatersources inmunicipalandindustrialapplications
[12-14],includingproducedwaters[6,7].Themodulardesign,smallequipmentfootprint,low
laborrequirements,andsuperiorproductwaterqualityallmakethemanattractivetreatment
option for producedwaters [2]. NF and RO are considered to be high-pressure membrane
processesastheytypicallyrequirefeedpressuresintherangeof100to1,000psig.Suchhigh-
pressure requirements arise fromthe relativelyhigh osmotic pressures that characterize the
feedwaterstotheseprocesses.
Pressure driven membrane processes utilize a semi-permeable membrane to separate
suspended and dissolved contaminants from a feed solution. Here, pressure is applied to a
feedwaterinordertoforcethewaterthroughthesemi-permeablemembrane,whichretainsthesalt(s)whileallowingwatertopassthroughasaresultofdifferencesindiffusivitybetween
thesoluteandwatermolecules.Becauseitisaseparation,andnotatreatment,processtwo
liquidprocessstreamsareproduced:i)acleandemineralizedproductwater(permeate)andii)a
rejector concentrated brine solution (concentrate).The operatingpressure inthemembrane
systemmustbegreaterthanthesolutionsosmoticpressureinorderforwatertoflowfromthe
feedsolutionandacrossthemembrane.Becausetheosmoticpressureincreaseswithincreasing
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salt concentration the pressure and pumping requirements will increase with TDS
concentrations.
The high-pressure feed pump is the largest energy consumer in high-pressure membrane
processes.Secondaryenergyconsumingdevicesincludetheconcentrateandpermeatebooster
pumps(ifrequired).Forsaltrejectingmembranesenergyconsumptionisdirectlyrelatedtothe
TDSconcentrationinthefeedwater,whichalsoultimatelydeterminestheachievablerecovery
ration(Qproduct/Qfeed)forthedesaltingprocess.Saltsimpartanosmoticpressurethatmustbe
overcomeinordertotransportwateracrossthemembrane.Thus,greaterfeedpressures,and
in turn pumping requirements, areneeded for higher salinity waters. Furthermore, practical
recoveryratiosforfeedTDSconcentrationsof36,000mg/Lare50%,withthisratiodecreasing
asTDSincreasesbeyondthisvalue.ThismeansthatforaproducedwatercharacterizedbyaTDS
concentrationof36,000mg/Lhalfofthewaterwillleavethetreatmentsystemascleanwater,
whiletheotherhalf(i.e.,theconcentrate)muststillbedisposedof.Thisisacriticalconcernfor
producedwaterswheretherawwaterTDSconcentrationmaybemanytimesthatofseawater
(TDS ~ 36,000 mg/L). Thus, concentrate disposal is a significant cost and environmental
consideration for desalting membrane processes. Membrane fouling is another important
consideration because it reduces membrane permeability and necessitates higher feedpressuresinordertomaintainadesiredpermeateflux.
Pictureofatypicalreverseosmosis(RO)desalinationtreatmentsystem
Theimportanceoffoulingpointstothesignificanceofimplementinganeffectivepretreatment
scheme inorder tominimizeenergy costs. This isa particularly relevantpoint for produced
watersastheymaycontainrelativelyhighconcentrationsofrecalcitrantfoulantmaterialssuch
as oils, greases, and dissolved metals [1]. Efforts to overcome fouling have led to the
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developmentof uniquedesignapproachesfor pressuredrivenmembraneprocesses,someof
whichareoutlinedbelow:
Vibratory Enhanced Membrane Process (VSEP): VSEP is a proprietary compactmechanicalmembranesystem (NewLogic Research,Oakland,CA) that canprocess
highsalinitywatersanddilutesludges.VSEPhassuccessfullybeenusedinmorethan
200 commercial-scale industrial applications treating extremely challenging source
waters(highTDS,highsolidscontent).Thesystemconsistsofaseriesofdisk-shaped,
flat-sheetmembranesattachedtoacentralshaft.Theshaftrotatesashortdistancein
onedirection,andthenreversesitself,atafrequencyof50to60timespersecond.At
the outer edge of the membrane disks, the amplitude of the oscillation can be
adjustedtobetween0.25and1.25inches.TheoscillatingmotionintheVSEPsystem
allowsNFandROmembranes to treat high TDSsourcewaters (e.g., 300,000mg/L
TDS)suchasthoseproducedbyshalegasactivities.Theoscillationreducesmembrane
foulingbyincreasingtheshearforcesandmixingatthemembranesurface.Thisaction
significantly reduces foulant deposition and the thickness of the concentration
polarizationlayerthatformsatthesurfaceofsaltrejectingmembranes.Bothofthese
actions would, if not controlled, contribute to a significant loss of permeate fluxthrough the membrane. The shear action prevents the formation of a continuous
scaleonthemembranesurface[15].Instead,themineralsnucleateandformcolloids
in the bulk solution. This allows the VSEP process to achieve higher raw water
recoveries, and treat waters having substantially higher TDS concentrations, than
conventionalROsystems.Furthermore,VSEPiscapableofprocessingsourcewaters
that have high concentrations of suspended solids and organic materials, thus
minimizing the amount of pretreatment requirements. Despite its promise the
application full-scale VSEP systems in produced water treatment applications has
been limited, likelyas a resultof comparatively high energyandcapitalequipment
requirements.
HighEfficiencyReverseOsmosis(HERO): AnotherROdesignapproachthathasbeendevelopedforincreasingtheachievablerecoveryratioforhighsalinitysourcewaters
is the high-efficiency reverse osmosis (HERO) process. Here, scale forming
compounds(Ca,Mg,Si)areremovedbeforetheROstepusingasofteningprocess.
Silicaprecipitationin theROprocessismitigatedbyoperatingata high solutionpH
(pH>9).CollectivelytheseeffortsreducemembranefoulingandallowfortheRO
systemtooperateathigherrecoveryratiosthantraditionalRO.Whileitispossibleto
achieve high feedwater recoveries (>90% in some cases), the consumptiveuse of
chemicalsissubstantialandthechallengesassociatedwithhighosmoticpressuresfor
highlysalinewatersremainsanissue.
Non-Pressure DrivenMembraneProcesses. Non-pressure drivenmembraneprocessesutilizemechanisms other than hydraulic pressure to transport water across a membrane barrier.
Examplesofnon-pressuredrivenmembraneprocessesincludemembranedistillation,forward
osmosis,andpervaporation.Whileeachoftheseprocessesaredescribedingreaterdetailbelow
it is prudent to point out that few of these processes are currently used in treating
produced/fracwaters.Forwardosmosishasperhapsreceivedthemostapplicationinfull-scale
settings.Someof theadvantagesand challengesthatareassociatedwith thesenon-pressure
driven processes arehighlighted in Table 2. Little cost data is available for thenon-pressure
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drivenprocessesthatarediscussedinthisreport;however,whereappropriatereferencewillbe
madeastotheuniquedesigncharacteristicsforeachprocessthatcanimproveorhindertheir
costcompetivenesstomoretraditionaldesaltingtechnologies.
Membrane Distillation (MD). Membrane distillation (MD) is a thermally driven separation
processthathasreceivedattentionasapossiblewaterandwastewatertreatmenttechnologyin
applications such as desalination and water reuse [16-18]. In contrast to processes like RO,
which utilize pressure as a driving force for mass transport, MDutilizes the vapor pressure
differenceacrossamembrane[18].InMD,thevaporpressuredifferenceisaffectedbydifferent
parameters [17]; however, the thermal gradient across the membrane is the primary
mechanism for mass transport. Water vapor is transported from the feed, which is at an
elevated temperature relative to the permeate side, across a hydrophobic microporous
membraneandintoacondensingmedium[17].ThereareavarietyofMDconfigurationswhich
maybeused[18];however,auniversalcriticalprocessparameteristhemaintenanceofthe
liquid-vaporinterface(i.e.,liquidwatercannotpenetratethemembranepores,whichrequires
theuseofadurablehydrophobicmembrane[19].
TheprincipleadvantageofMDderivesfromthefactthatitisathermally,andnotpressure,driven separation process. Therefore, MD does not need to overcome the high osmotic
pressures that characterize producedwaters. For this reason,MD is an attractive treatment
technology for produced waters because it is not osmotically limited like pressure driven
membraneprocesses.Additionally,MDrequiressignificantlyloweroperatingtemperaturesand
thus has lower energy requirements relative to mechanical evaporation processes. It is
importanttobearinmindthoughthatawasteheatsourcemustbeavailableinordertoallow
theMDprocesstofunction.Intheabsenceofaheatsourcetheenergyrequirements,andthus
thecosts,associatedwithMDcan increasedramatically. Finally,because non-volatilesolutes
cannotbetransportedacrossthemembranebarrierinaMDsystem,itis capableofachieving
near100%rejectionofdissolvedsaltsandminerals[17].Forthesereasons,MDisa promising
technologythathasprogressivelygainedattentionas a treatmentalternativeforhigh salinity
sourcewaters[17,20].
ForwardOsmosis.Forwardosmosis(FO)operatesontheprocessofnaturalosmosisinwhich
waterflowsfromanareaoflowsaltconcentration,acrossasemi-permeablemembrane,toan
areaofhighsaltconcentration inanattempttoreachanequilibriumstate(balancingoutthe
osmotic pressure difference between the two solutions. FO is sometimes referred to as
engineeredosmosisasanosmoticagentisusedtodrawwaterfromasalinefeedwater,suchas
producedwater,intoadraworcapturesolution.ThetwomostcriticalcomponentsinanFO
systemaretheosmoticagentandthemembrane.Tobesuccessfultheosmoticagentmustbe
highlysolubleinwater,beeasilyrecovered,andimpartahighosmoticpressurewhendissolved
insolution.Themostpromisingosmoticagentsincludevarioustypesofammoniasaltsbecause
they can be relatively easily recovered from solution and reused. There are a few FOmembranesandsystemscurrentlyonthemarket(seee.g.,HydrationTechnologyInnovations);
however,aninherentchallengewithFOprocessesisconcentrationpolarization.Concentration
polarizationcanoccuronboththefeedandpermeatesidesofthemembrane,aswellasinthe
membrane interior (internal concentration polarization). All of these types of concentration
polarization act to reduce the osmotic pressure gradient between the feed and permeate
solutions resulting in a reduction in the permeate flux rate. This is particularly challenging
becauseitisdifficulttoovercomeoperationally.Forexample,inpressuredrivenprocessesitis
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possibletoincreasethehydraulicpressurethatisappliedinordertomaintainaconstant flux
ratewhileaccommodatinglossesinfluxasaresultoffouling.Conversely,increasingtheosmotic
pressuregradientrequirestheadditionofgreaterquantitiesofosmoticagentand/orincreased
mixing at themembrane surfaces. Regardless of theaction taken concentration polarization
posesasignificanthurdletothewidespreadapplicationofFOintheproducedwatersector,
becauseofthealreadylowfluxratesthatcharacterizethisprocess.Nevertheless,advances in
membranematerials,processdesign,andnewtypesofosmoticagentsarepromising.
Table2. Advantages andchallengesassociatedwith different non-pressure drivenmembrane
processesinproducedwatertreatmentapplications.
Process Advantages Challenges
MembraneDistillation
Lowpumpingrequirementsresultinginlowenergy
footprintassumingwaste
heatsourceisavailable Capableoftreatinghigh
salinitysolutions(TDS>
50,000mg/L)
Requireswasteheatsourcetodrivemasstransport
Lackofcommerciallyavailablemembranes
Susceptibilitytoporefloodingfrommembrane
foulingresultinginlackof
ionrejection
Largelyunprovenatfull-scaleinstallations
ForwardOsmosis
Withproperselectionofosmoticagentitiscapable
oftreatinghighsalinity
solutions(TDS>50,000mg/L)
Pumpingrequirementsarelowasmasstransportis
drivenbydifferencesin
osmoticpressure
Recoveryofosmoticagentcanbetechnicallyand
economicallychallenging
Concentrationpolarization(internal,external)
dramaticallyreducespermeatefluxrates
Comparativelylowfluxratestopressuredriven
membraneprocesses
Fewfull-scaleinstallationsandlimitedcommercially
availablemembranes
Ion Exchange. Ion exchange (IX) is a process in which ions are exchanged between an ion
containing solution and a bed of synthetic resin beads (adsorbent) presaturated with
noncontaminantions,suchassodium(Na+),chloride(Cl
-),hydrogen(H
+),orhydroxyls(OH
-)[21].
Using IX it is possible to selectively remove nitrogen compounds, hardness (i.e., water
softening),andmonovalentionslikesodiumandchloridefromaqueousstreamsandhaswidely
been applied inmunicipal, industrial,and residentialapplications.While IXmaybeused ina
widerangeofapplications,watersofteningwithgelresinsremainsasthemostwidespread.In
mostcasesIXisrestrictedtoapplicationswhereultrapurewaterarerequired(industrialmakeup
water) or for water softening (residential applications); however, in some instances it is
applicable to the treatment of more challenging feed streams such as produced waters.
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Specifically,theapplicationofIXfortreatingproducedwaterswillbedependentontheionic
composition of the feed stream as it ismost appropriate for waters composedprimarily of
bicarbonate ions (HCO3-).Thisformof treatment is termed strong base IX, where the OH
-is
exchangedforthebicarbonateion.InthesecasesIXiscapableofeffectivelytreatingproduced
waterstohighstandards.UnlikemembraneprocessesIXdoesnotuseasemi-permeablebarrier
to separate the dissolved salts and minerals from water. Instead, IX is both an adsorption
processandachemicalreaction.Itresemblesadsorptionbecausesolidparticles(resin)areused
and regenerated, while the chemical reaction specifically applies to the regeneration of the
resin.
IXrequiresarelativelyhighqualitysourcewaterthatis freeofparticulates, foulantmaterials
and other competing ions for the exchange sites in the resin. Therefore, its application has
primarilybeenrestrictedtothetreatmentofCBMproducedwaters,whicharerelativelyfreeof
contaminantsoutsideof theaforementioned bicarbonates, andwaters that have undergone
extensivepretreatment.TheprimarycostsassociatedwithIXaretheresin,regenerationofthe
resin, capital equipment (pumps, motors, IX columns), and disposal costs associated with
disposaloftheregeneratingsolutionfortheIXresin.
MechanicalEvaporation.Mechanicalevaporationprocessesareinmanycasestheonlysuitable
optionfordisposingofhighTDS(TDS>50,000mg/L)wastewaters.Theseprocessesinvolvethe
evaporationofwaterthroughavarietyofmeanstoultimatelyproduceasolidscakecontaining
allofthedissolvedsolidsthatwerepresentintheproducedwater.Theseprocessesareenergy
intensiveandthusareassociatedwithsubstantialcapitalandoperationandmaintenancecosts.
However,as previously stated they arein manycases theonly optionwhen disposalof high
salinity produced waters is required. Summaries of some example mechanical evaporation
technologiesthatmaybeusedintreatingproducedwateraregivenbelow.
Multi-EffectDistillation(MED):MEDisanestablishedprocessfordesaltinghighsalinitywaters (TDS > 36,000 mg/L). In a MED system, water is boiled in a sequence of
evaporators,eachheldatalowerpressurethanthelast.Eachevaporatorintheseriesis
calledan"effect".Becausetheboilingpointofwaterdecreasesaspressuredecreases,
the vapor boiled off in one vessel can be used to heat the next, and only the first
evaporator (the one at the highest pressure) requires an external source of heat.A
reduced pressure in the vapor space of the first evaporatormust bemaintained to
account for the difference in the boiling points of pure and saline water. Another
requirementtomaintainreasonableheatexchangebetweenthepipescontainingthe
condensingsteamandthosewiththeboilingproducedwater,thetemperatureof the
producedwatermustbeseveraldegreeslowerthanthatofthecondensingsteam.MED
systemstypically operateata lowtemperatureof 71.1Cand a high temperatureof
110C. Operating at lower temperatures limits corrosion and these systems can be
constructedoutoflessexpensivematerials.Theamountoffreshwaterproducedperunit amount of heating steam increases almost proportionally with the number of
stages.While in theory,evaporatorsmay bebuilt with anarbitrarilylargenumberof
stages,evaporatorswithmorethanfourstagesarerarelypractical.
Multi-Stage Flash (MSF)Distillation:MSFdistillssalinewaterbyflashingaportionofthe feedwater into steam inmultiple stages. InMSF the produced water is heated
underhighpressureinordertopreventboiling,untilitreachesthefirstflashchamber.
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In the flash chamber the pressure is released and sudden evaporation or flashing
takesplace.Flashingcontinuesineachsuccessivestage,becausethepressureislower
asyouprogressfromonestagetothenext.Thewatergainsheatasitpassesthrough
eachstagebycondensingvaporsthataregeneratedbytheflashingprocess.Thesteam
iscondensedontubesofheatexchangersthatrunthrougheachstage.MSFtreatment
systemstypicallyutilizeawasteheatsourceinordertoreducetheenergyconsumption
byone-halftotwo-thirds.
MechanicalVaporCompression(MVC): MVCisgenerallyusedforsmall-andmedium-scale (Q = 0.005 to 0.5 mgd) desalination systems. The heat for evaporating the
producedwatercomesfromthecompressionofvaporratherthanthedirectexchange
ofheatfromsteamproducedinaboiler.Theboilingpointofthewaterisreducedby
reducingthepressurethatisappliedtoit.Twomethodsareusedtocondensethevapor
soas toproduce enough heat toevaporate incomingproducedwater: amechanical
compressororasteamjet.Themechanicalcompressorisusuallyelectricallydriven.
Mechanical zero liquid discharge (ZLD) systems also fall under the category ofmechanical
evaporation systems and include thermal evaporators, crystallizers and spray dryers. ThesetreatmenttechnologiesarecommonlyusedincombinationwithROsystemsinordertoachieve
azeroliquiddischargestatus(i.e.,noliquidwastestreamresultingfromtreatingtheproduced
orfracwater).Thecapitalandoperationalcostsforthesethermalsystemsaretypicallyhigher
thanforthedesalinationmembranefacilityduetotheextensivemechanicalsystemsandexotic
alloy materials required. In addition, the energy costs associated with the evaporation
processingaresignificant.Zero liquiddischargesystemsultimatelyreducetheconcentrateor
producedwatertoasolidproduct(crystallizedsaltsandminerals)forlandfilldisposal.Insome
cases, the water vapor is recovered. A summary comparison of mechanical evaporation
processestoROisgiveninTable3.
Table3.Comparisonofperformancestatisticsformechanicalevaporationandreverseosmosis
(RO)desaltingtechnologies.
ProcessEnergyUse
a
(kWh/1,000gal)Methodof
OperationSystem
Recovery(%)Relative
CapitalCosts
MSF 58 Steam(heat) 1020 High
MED 29 Steam(heat) 2060 MediumtoHigh
MVC 3053Compression
(heat)3599 High
RO 823 Pressure 3555b LowtoMedium
Notes:
aCombinedelectricalandequivalentthermalenergy. bRecoveryratiosareafunctionofthefeedwaterTDSconcentration.Recoveryratiosincrease
beyond50%asthefeedwaterTDSdecreasesbelowapproximately36,000mg/L.
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11
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In an ever changing world with finite resources, it is now more critical
than ever for America to tap into its existing energy resources: oil and
natural gas. Domestic energy development activities are setting us free
from the reliance of foreign imports. But with this freedom comes achallenge: how to manage the vast quantities of water that are
produced and consumed in the process.
Meet the Nexus Group. A team of forward thinking specialists who havededicated the last 10 years to researching cutting edge technologies
and solutions to this very issue.
Our mission: To bridge the gap between a sustainable environment and
a sustainable economy, one drop at a time.
JONATHAN A. BRANT, [email protected]
NexusGroupSolutions.com
office: 307.766.5446
cell: 307.275.2677
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the agreement between the client and The Nexus Group under which this work was completed. The report may not
be relied upon by any other party without the express written agreement of The Nexus Group.
Any recommendations, opinions or findings stated in this report are based on circumstances and facts as they
existed at the time The Nexus Group performed the work. Any changes in such circumstances and facts upon which
this report is based may adversely affect any recommendations, opinions or findings contained in this report.
The Nexus Group does not make any warranty, express or implied, or assume any liability or responsibility for the
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