Emulsion Problems

8/2/2019 Emulsion Problems

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.+ November1995L.

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P. M. Hennessey, andM. Neuman, Otto H. YoI"kCO., Inc., Parsippany, NewJersey, B. A. Kalis, Otto H.York Co., Inc., Houston, Texasand G. Hellinx, Otto YorkN.V., Merksem, Belgium

Reprinted from HYDROCARBON PROCESSING@, November 1995.Copyright @ 1995 by GuIfPubIishing Co., Houston, Texas.AlI rights reserved. Used with permission.


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PROCESS TECHNOLOGY ILOSS PREVENTIONli'

Case histories show howto cost-effectively separatetwo immiscible liquids

quickly to be cost-effective.The simplest way to coalescedispersion s to just letit sit in place. In most cases, t willsooner or later coalesce, ettle out and forro two distinctlayers with the help of gravity. Seldom s this process aneconomicalsolution. To speed up the process, he disper-sion is usually forced to flow through porous media withhigh specific surface and perhaps with definite wet-abil-ity characteristics. Small dispersed droplets are then con-verted into large ones which quickly settle out.P. M. Hennessey and M. Neuman, Otto H. York Co.,Inc., Parsippany,New.Jersey,B. A. Kalis, Otto H. YorkCo., nc., Houston, Texas, and G. Hellinx, Otto York N. V.,Merksem, Belgium

, hat are efficient and cost-effectiveways to sep-arate the dispersion of two immiscible liquids?A common esult from processinghydrocarbons,dispersionsexist when he dropletsof one iquid become is-persed nto another. Because he droplets are small [downto 1 .um (micron}), molecular forces suchas interfacial ten-sion, viscosity, density, temperature, pH, etc., contraI therequired time for the two liquids to separate.Old methodsused arge settler vessels nd allowedgrav-ity t()merge he droplets nto sufficientsize or separation. bbemore competitive,operatorsare ncreasingprocess apac-ity and consequently,ncurring more problemswith disper-sions rom higher throughputs. Thus, many operatorsmustfind processmethods that acceleratehe dispersionsepara-tion process and/or preventdispersion formation. Usingmethods such as liquid-liquid coalescingmedia s one wayto convert small droplets nto large ones hat quickly settle.

Benefits. Gravity separation, the first andstill commonmethod,only works effectivelywhen a arge retention ime isprovided.With heavy lows, very large vesselsare required.Some horizontal vesselsare 15 i to 20 fi in diameter andare 45 t to 60 t longoPurchasing a gravity separatorcan bea major capital nvestment. However,when using speciallydesignedcoalescermedia, he vesselsize can sometimesbereduced o one-third of the original designoResult: A sub-stantial reduction n required capital investment.Much ofthe refining industry, however,has arge gra. ~!ity-separator vesselsalready existing in their plants. Wit'rrl'today's separation efficiency requirements, many gravityseparators donot meet exit standards. Retrofitting thesevesselswith a coalescer lement designedfor he particularoperating conditions can have many economicand envi-ronmental advantages. The refining and gas processingindustries provide significant examples on the benefits ofusing well-designedcoalescingmedia.Case history 1. Problem. A medium-size refinery inSouthAmerica producedjet fuel with a high haze number,indicating a stable water emulsion which required settlingin product tankage. Five days of settling were required toclear up the haze and meet he clear & bright specification.One ofthe four jet fuel tanks (100,000 bbl each)was con-stantly used o settle out water. The additional inventorycost (on a:V.S. basis) o keep a 100,000bbl/yr in a settling

Widespread. The presented casehistories show how coa-lescers were effectively used in refining and gas-process-ing applications. Dispersions can be expensive problemsthat affect product quality suchas haze njet fuel; they canbe an environmental problem such as hydrocarbonemul-sions present n wastewater; and they canbe an operationsproblem such as solvent osses n an amine unit. However,by using these examples,operators can ocus on how to cutoperating costs, mprove product quality and reduce pro-cesswastes especially when handling dispersions.Old problem. Coalescing s one ofthe most essential andwidely used processesn the hydrocarbon rocessingndus-try (HPI). The separation of two immiscible liquids is abasic operation n refineries, iquefied gasprocessing lants,petrochemical acilities and many sectorsof the chemicalbusiness. It is also used frequently in environmental pro-cesses.Water dischargescan dependon t; jet fuel dependson it; alkylation increasingly dependson t; and for desul-furization of liquefied gases, t is necessary. he chemicalindustry has various specific uses for coalescing hat aretoo numerous to mention. Anytime droplets of one liquidbecame dispersed in another, they must be separated

HYDROCARBON PROCESSING I NOVEMBER 19952


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existing veRseI,a cylindrical coalescer made from com-pressedmicrofibers of glass was used o provide a high sur-face area, low flow velocities and maximize the usage ofthe existing vessel'svolume...

Results. After the initial startup, the coalescer educedthe water content down to 75 pprnand the haze n the dieselproductdisappeared.Delays n dieselblendingwere reducedby one day. Furthermore, additionaltankage becameavail-able which was previously used for water settling. Savingswere $100,OOO/yrU.S. basis).Case history 3. Problem. Another large refinery on theU .S. Gulf Coast was caustic treating FCC (fluid catalyticcracker) products to reduce he mercaptan sulfur content,and then water washing to remove excesscaustic. BothFCC products were used in gasolineblending. The amountof caustic carryover was too much for the water-wash acil-ity to handle. Therefore, the finished gasoline productswere often hazy and high in sodium contentoSolution. Researching he facility's performancehistory, itwas determined that the coalescerson the caustic drumsneededa specific internal geometry. Pilot tests indicatedthat the caustic nfluenced the interfacial tension makingthe emulsionvery stable. Laboratory ests ndicated that thefacility was getting 3,500 ppm of caustic carryover and2,500 ppm water carryover from the water-wash step.Results. The refinery installed in the caustic stage,a tightlyknitted wire mesh coalescer stainless steel media) and aproprietary media of verticallaminations of knitted wiremesh and fiberglass in the water-wash stage. The causticcoFl-tentram the caustic wash stepwas reduced o 100ppm.The balancewas easily emovedwith the water-wash tep.Attheoutlet ofthe water wash, he water contentwas reducedto 100ppm. As a direct result, both the hazinessofthe gaso-line and the sodiumcontentwere emoved rom the product.AdditionaI benefits. Becauseofelevated VOC (volatileorganic compound)evels in the plant atmosphere, he EPAhad restricted the amount oftimes a gasoline tank can bedrained per day. Now that most ofthe water was removedby the coalescer, he gasoline tanks required very littledrainage. This allowed the facility to meet he VOC limits.

Fig. 2. The concourse of the coalescence process of several dropletsapproaching an interface.mode would be approximately$200,000/yrassumingan 8%interest rate. The loss of one tank severely imited ship-ping flexibility of the jet-fuel blending operation.(~) Solution. Engineersevaluated he process hat produced~e water emulsionand reviewed he probability of contam-inants that could nfluence the interfacial tension. A bottletest was dane with the hazyproduct o md the emulsion's et-tling rate. Laboratory analysis determined that the freewater in the jet fuel ranged from 5,000 ppm to 1,000 ppm(parts per million). Based on the process conditions andempirical data, a coalescer lementconstructed from a co-knit ofmetal and fiberglass was designed.An existing 8-ftdiameter separatorwas retrofitted using this coalescer.Results. The coalescerperformed well, removing thewater down to 50 ppm. Thus, thejet-fuel was clear & brightand ready for immediate shipping.Economic advantage. After a month of successful per-ation, the large jet-fuel tank used for "water settling" wasput back as a Bales ank. Reduced nventory fees, loweroperating costand no shippingdelays due o water settlingtotaled around $450,000/yr.

Case history 4. Problem. A European refinery wa8 108-ing about2 vol% ofDEA (diethanolamine)801ventwith theCase history 2. Problem. A large refmery on the Gulf CoastofMexico increased its diesel production. The vertical gravityseparator was now a limiting factor. Insufficient water remova!made the diesel fuel hazy and created blending delays.Solution. A laboratory analysis by a consulting technicalstaff determined that the dieselcontained about 3,000 ppmof free water. Economicsprohibited replacing the existing~parator. Th emovehaze rom this dieselwith a low nter-~lcial tension, a large amountof coalescing reawas needed.It was established that a velocity less than 2ft/min. wasneeded o effectively emove he haze. A full-diameter coa-lescer in the existing vesselwould not permit Bucha lowyelocity. Th overcome he velocity problem and retrofit the Figo 30 The total force of adherence is a functon of angle 9.

HYDROCARBON PROCESSINf.; I NOVEMBER 1995 3


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surtI as catalysis or distilIation. A modern coalescer ele-ment may alIow capacity increases of 50% to 200% byenabling superficial velocities to increase along withimproved separation. ~ically, the elements can effectivel~' '\remove aqueous solutlons from a hydrocarbon to below 2iW!ppm and vice-versa. Such performance means maximumrecovery ofvaluable hydrocarbons and minimaltreatingcost for the separated wastewater.With increased plant capacity and ever stricter environ-mental regulations, the separation efficiency ofwater fromoil and Gil rom water has becomean important control aspectoIn some plants, the ability to efficiently separate water fromhydrocarbons determines if the plant has to shut down ornoto A high efficiency coalescer designed for its particularapplication can effectively separate water from oil, leavingonly traces ofwater behind. Then further processing ofthehydrocarbon can be done immediately without further delays.

Fig. 4. Horizontal coalescer-vessel geometry where the dispersedphase has the higher specific gravity.

LPG (liquefied petroleum gas) product from an absorptiontower and wanted to reduce his lossto 100 ppm carryoverwith the LPG.Solution. Application engineers eviewed he plant draw-ings and recommended nstalling a coalescerat the top ofthe absorber. As an alternative, the coalescercould alsohave been nstalled in a downstreamLPG separator. Theabsorber ower diameter was large enough or a coalescerand would allow for a 1.5 t/min. vertical velocity to ensurean efficient separation. A co-knit of wire and ultra-finepolyester fiber which is specifically deSign1d for vertical.flow coalescing applications was installed.Results. After installation of the co-knit unit, the DEAcarryover into the LPG product was reduce~ o less than100 ppm. This constituted an approximate savings of$ 140,000/yr U.S. basis) n DEA solventmakeup.

Using modern coalescers in lhe HPI. When gravityseparation is inadequate, an oil emulsion is drained with thewastewater from a gravity settler. Especially in light dis-tillate settlers, the oil entrained in the water has a highvapor pressure and vaporizes as the wastewater is drainedthrough the sewer system to the wastewater treating area.Since there are dozens ofthese settlers in the plant in theforro ofreflux drums, condensers and three-phase separa-tors, the facility's fugi tive VOCs leveI can increase sub-stantially. The garoe is true for water left in hydrocarbonproducts. Water that settles in tankage is drained to watertreating facilities. While it is draining, part of the hydro-carbon vaporizes adding to the ambient VOC leveI.Industrial interestto retrofit separators with coalesce(~I.)in any process vessel containing hydrocarbons and wate~has tripled in the last two years. A coalescer can produce asharp separation between the hydrocarbon and water.Therefore, the wastewater from a separator may containlevels of oil as low as 20 ppm to 30 ppm.Results. Plant VOC levels can be markedly improvedif coalescers are used where water is separated from

hydrocarbon product.Reduce wastewater treating casto A correspondingadvantage of coalescers is the ability to reduce the hydro-ncreased capacity and efficiency. As the HPI contin-ues a capacity consolidation and gears up to comply withmany environmental restrictions, older less efficient plantsare being phased auto Those remaining must upgrade theirprocesses to:.Pick up the lost production capacity of closed plants.Comply with ever stricter environmental rules.Expand plant capacity to satisfy annual growth.Cash-flow availability for capital investment is gener-ally limited. Therefore, industry has sought increasingcapacity by modifying existing equipment. In the '90s,revamp and debottlenecking projects are in full swing. Plantcapacity requirements are often increased a$ much as 40%to 50% with traditional gravity settlers experiencing a sub-stantial increase in liquid flow. Retention time now becomesinsufficient and separation is incomplete, thus complicat-ing and overloading the downstream processes. Removinghaze from jet fuel is a prime example of the benefits of usingmodern oalescingquipment.A review of the databases for refinery, LNG (liquefiednatural gas), LPG andpetrochemical p.lants will show asurprising number ofliquid-liquid separation vessels thatare frequently ignored in favor of more e~otic processes

-Fig. 5. Horizontal coalescer-vessel geometry where the dispersedphase has the lower specific gravity.HYDROCARBONPROCESSINGI NOVEMBER 19954


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amaunt af water in the reflux will flash as it enters thefractionator and leave salt deposits on the trays..Flashingupsets the vapor traffic at the top of the fractionator andhinder its efficiency. When a coalescer s retrofitted in areflux aTum, t can ensure maxirnum water removal fromthe hydrocarbon. Low-water content in the reflux stabi-lizes tower operation and reduces costly upset conditions.Reduced corrosion. In a refinery, there are severalareaswhere inadequate iquid/liquid coalescence ausescorro-siGnoCorrosion s another consequenceof sulfur and car-bonic salt depositionon fractionator trays. In other caseswhere caustic s present n the water and settles to the bot-tom of a carbon steel storage tank, the bottom plates mayslowly corrode.Caustic may algo carry over from mercaptantreaters or caustic wash acilities. To avoid carryover,a coa-lescer vesselcould be installed in product pipelines thatwould removecaustic down to 50 ppm.Improved product quality. Water and/or causticentrainment in a hydrocarbon can cause product qualityproblems. nstalling coalescers as enabled many refiner-ies to meet oday'smostdemandingspecifications ncluding:..Clear & brights in jet fuels

..Haze in distil1ates..Sodium content of gasoline.Other potential applications. Caustic treaters for dis-tillates are used o convert mercaptan sulfur to less obnox-ious sulfur forms. In these treaters, somecaustic remainswitlrthe hydrocarbon product and must to be removed.Thus, most caustic-wash acilities are followed by a waterwash. If removal of caustic entrainment is insufficient,additional water must be used in the wash step. Byretrofitting a coalescern the caustic-washand water-washsystems,operations are improved:.A higher quality distillate is produced.A reduced quantity of wash water is used to removecaustic.Less wastewater s generated.Feed to the alkylation plant often contains substan-tial emulsified water. This water dilutes acid in the reactorand adds to corrosion throughout the planto Whenretrofitting a coalescernto the feed separator, he amountof acid make-up s reduced, owering operating cost.

"'!C "j!''

carboncontentofwastewater.This can meana direct reduc-tion in treatment cost since a major function wastewatertreatment system is the removal of hydrocarbQnsbeforethe water is discharged to rivers or bays. Ir a wastewaterstream has a high concentration of solids, hese ~eed o besettled out or filtered to avoid pluggingofhigh-efficiency orfibrous coalescerelements. An API separator prior to thecoalescer s sometimes useful for this purpose.Improved product blending time. When products/#imponents have a low water content becauseofupstream.alescers, the product blending time is subsltantiallyreduced. This also yields reduced inventory CO$t, voidsdelays in shipping and lowers overall operating tostoIncreased solvent recovery lowers operating costs.In a liquid LPG solvent-treating lant, solvents uchas MEA,DEA or MDEA (monoethanolamine, diethanollamine ormethyldiethanolamine) are used to removeH2S from LPGin an absorber ower. The H2S-richsolvent s then regener-ated. Hydrogensulfide is sent to a sulfur recoveryunit andlean solvent recycled back to the absorber ower. The sol-vent, which is an aqueoussolution, can form a staibleemul-sion that is lost with the sweetened PG. Likewis, naturalgas liquids emulsify in the H2S-rich amine solu-tions. The absorberand regenerator owers can beeither packed or trayed and designedas gas/liquid 1.or liquid/liquid extraction columns. Speciallydesignedco-knitcoalescers, ith a uniquegeometry,have been etrofitted in the top of such owers or na downstream separatorand have achieved ecovtries of95% ofthe solvent previously ost.Improved fractionation tower operationMost fractionation towers, like crude or vacuumtowers, have an overhead reflux drum or three.phase separator. These vessels are algo found inthe FCC and cokeunits. In the top part of a three.phase separator, vent gases are separated fromliquids. In the bottom section, the hydrocarbon.;oduct is separated rom water. When unit capac.~y is increased, he retention time becomesnsuf~icient for goodseparation.Results. More water is carried over with thhydrocarbon product and refluxo Even a smal

Nominaltarget slze.1 n. to4 n.(spacing) Both100-500~m 80th

MediaParallel ndcorrugated latesandvanesKnlttedmeshmetal,plastlcandb)-componentCo.knlls 1wireandmulti-strandfibers

12-351m Both

Compressedmlcr-fibersofglass,plasticand metal5,-35 1m 80th

HYDROCAJt.BON PROCESSING / NOVEMBER 1995 5


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recirculate with the solvent. Over time, the hydrocarbonconcentration ncreases o a leveI where it acts as a foam-ing initiator. Many plants have a separator tank or hori-zontal vesselwhere they can skim off someof the hydro-.carbon using gravity separation. Since gravity separatijis not very efficient, part ofthe hydrocarbons continue tobe recycled. In addition, as hydrocarbons are skimmedoff, someof the emulsified solvent will algo be removed,thereby addingto solvent losses. To avoid liquid hydro-carbon buildup in the solvent, an efficient coalescer an beinstalledupstream ofthe regenerator column.

Fig. 7. Three-phase separator configuration divides a vapor from twoliquid phases.

COALESCERS-ART OR SCIENCEFormation ofliquid-liquid dispersions is a frequent occur-rence in the HPI. Separation difficulty varies with the dis-persion type which is characterized by the droplet size. If auniformdispersion is allowed to settle, it will usually fonn twodistinct layersrather quickly from the primary dispersionwhere droplets will have diameters greater than 100 ~m.After the definire interface is fonned, both layers will remaincloudy and coalescencecontinues at a much slower raro withthe secondary dispersion where droplets are much smaller.Traditionally, liquid-liquid coalescencehas been accomplishedby equipmentdesigned for gravity settling. Primary disper-sions are usually separated adequately in the wastewatertreatment plant's API separator, and specially designed ver-tical and horizontal vessels such as the reflux drum of asteam stripper or a caustic wash tower.In the '90s, process engineers must design and specifyliquid-liquid and three-phase separators to meet evertougher environmental regulations, to reach increasinglystringent product specifications, and to handle higher prat 'iduction rates. Whether the problem is the biological oxpJ..gen demand leveI in the biological treatment system, timelost while ahaze settles out in a diesel storage tank orthe plant's sudden demand for drier seal-flush oil, thelikely problem is the remaining entrainment as a sec-ondary dispersion. Various coalescing media have beenused for over 30 years in the HPI as flow-through devicesto break these fine stable emulsions.High efficiency media. Proprietary media availabletoday uses a wide range of target diameters, surfacechemistries and geometric structures tailored to specificapplications. The more efficient the media is, the less capac-ity and the higher pressure drop it has. Table Ilists mostmedia types on the market in ascending order of efficiency.Fundamentais of coalescing. Separating he dispersionof one mmiscible liquid in another is usually consideredmore of an art than a science.Becauseof the many vari-ables, most research has concentrated on the practicalaspectsof achieving a separation under definite, specificconditions.The literature contains ample data for specificsituations, but little theory that can be applied to practi-cal problems on a broad range of applications. This is whymany considercoalescenceo be a black box operation andindeed practical experience s vital to an effective and eco-nomic solution for most industrial coalescer roblems.-Nevertheless, progress has been made and databasesbuilt,. based on specific designs for specific applicationAl)Conslder the general modes of coalescence. For a norm~dispersion, two major steps are involved in the separa-tion. The first step s truly coalescence,wherein very finedroplets physically join one another to forro larger drops

In MfBE and TAME units, wash water s used n boththe main and methano1 recovery extraction columns toremove impurities and recover methanol. A water emul-sion can carry over to the downstream reactor from bothextractors. Water causes side reactions aI1ld 0wers heamount and purity of the MTBE and TAME products. Toavoid this, coalescers an be added n the top ofthe extrac-tors or downstream separators. IFrom a crude desalter unit, a 1argeqti1antityof oi1ywater is discharged. Using a coa1escer nitto remove oi1from this water wi11substantia1ly educe h~ costof treat-ing a major refinery wastewater source.Amines. Treatments of natural gas with amines forsweetening and glycol for drying can suffet from seriousfoaming problems. Both absorption and regeneration ow-ers can generate roam on trays or in packing. Foamingresu1ts rom the formation of stable bubble$. The volumeto weight ratio for these stable bubb1ess ~igh, resultingin the gas carrying the roamoverhead. n adtlition, when abubb1e ursts, the shattered film createsvery fine dropletsthat are difficu1t to collect. Excessive oaming in the sol-vent plant increases operating costs n severa1ways:.Foaming in the absorber tower decreases he effec-tiveness ofthe solvent. Consequent1y,he hytlrogensulfidecontent in the overhead gasescan ncrease..Foam and resu1ting fines reaching thei mist elimina-ror reduces efficiency and thus increases solvent entrain-ment loss..Temporary feedrate eduction o "drain" tpamwiIl resultin lost production..Lost operating time to correct he prob]em..Foaming in the regenerator tower incteases the sol-vent entrainment losseswith the hydrogenBu1fide asses.Solvent contaminants are major contributors to foam-ing. They can be condensed ight hydrocatbons, organicacids, water or traces of treating chem~ca1s rom anupstream processoHydrocarbon contamin*nts in the gasfeed may not initia1ly cause any foaming. Mowever,sincethese light hydrocarbons remain as a 1iqrid, they may

HYDROCARBON PROCEHHING / NOVEMBER ] 991\6


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gravity settling. The allowable rate ofrise ofthe dispersedphase s foundas the value where the buoyant forces onan ojJ.droplet of a given diameter equals the drag forcesexertedby he viscosityof the continuouswastewaterphase.A derivation from Stoke's Law enables the calculation ofthe rate ofrise:v- g (~ Pu,-Po)D2

Where~

which can then set e quickly by gravity. F b ous or gran-

ul ar me di a mar be u se d to pro mo te thi s st ep Th e secon d

stepinvolves theselect on of ow pat emst promote an

i , e~ tv e c ol lec ton.ofhes e large drops into t e l iquid- .q-

' . ld r nter ace. Physlcal and chem cal proper s of he sub-

s tanc es w i l l determ ne the tm e needed for c m plete s ep-

arat on and, consequent y, govern equipme t sizes and

theeconomcs.

A very fne dispersion can have drople s between 1

to 10jlm or even smaller Frequent y, the eye detects these

f ne d is pe rsi on s o nl y a s a sl ig ht c lo ud in ess o r a h aze w hi ch

mar beextremely stable, evento the extent where there

mar b en o ap pre ci ab lesep ara to n af er se ve rai l da ys o fse t-

t ing. The inherent proper es of both imm scible con-

sttuents afects both therate and degree o~ separaton.

Proper es of nterest foreach phase are: densf y, viscosity,

sur ace tension, temperature, degreeofdispersion (par cle

size) and pH. Proper es of heemulsion that come into

piar are inter ~c. al tension, rat o of the cons f ituents, thex te nt o fs ol ub ul ty o f he p ha se sa nd pr es en e o fsu r ac -

t an ts o r g el li ng a ge nt s.

In recent years, much work has been don to descr be

and quant fy the very complexmechanism ofcoale.cence.

The objec tv e is to enable the des ign of indus t ra l equip-

ment to accomplish the necessary separat c:>ns. Several

researchersl.2 have contrbute d to an understandingof

these phenomena based on droplet gizes. ~owever the

de te rm nat on of d ropl et g ize s d istr but on s is too t me con -

sum ng for industr al processes.3 Indeed,most designs are

not based on theoret cal assumpt ons, but ather from

exper mental invest gat onsof per nent m tures. Most

(,heoretcal investgatons are based on cal ulatons of

\ ~roplet gizes for pure components.But the p edicted rate

ofcoa le scen ce fo r a n i ndu st ra l syste m was si x t mes la rge r

than the exper mentalvalue.4 Most predict on methods

are based on a mean droplet size, but since he extent of

separat on is a funct on of small droplets, a droplet-size

distr but on is a more rat onal approach, eve though too

cost y formost applicat ons.

The degree of dif cul. y in making a separ t on is usu-

a ll y a f un ct on o f he e mu ls io n' s s ta bi li ty . V er y s ta bl e e mu l-

sions can result from a number of causes. Sur actants or

emulsifying agents can have signif cant ef ef s on stabi-

lizat on even in quant tes which are dif cult to detect by

analyt cal methods. A m nor dispersion ofveI y f ne solids

wi ll t end t o sta bil ize the in te racia l f lm whi ch m ust bre ak

for coalescence to take place. This f lm will 111so be more

stableif t has gelat nous proper es due to the viscosity of

the const tuents, f ne solids, electr cal charges or chem -

cal proper es For two droplets to coalesce or for a droplet

to join its mother phase layer the f lm sepa at ng them

must hinout (or drain) unt lit is so thint at it f nally

ruptures allowing two discrete droplets to become one

larger droplet Researchers5 havestudied the phenomena

l e ad in g t o t h e r up t ur e o f h e t h in f lm a nd h v e p ro p os ed

mathemat cal treatments for var ous situat o s. In F g. 1,

a single droplet is joining at the inter ace at d in F g. 2,

a swarm of droplets are approaching the inter ace. Both

r g uresdepic~ droplets that have a lower spelcif c gravity

Whan the contnuou s phase.

High-efficiency elements handle secondary disper-sions. To improve coalescence over that which can beachieved by gravity alone, various fibrous media includingfelts of polyester and polypropylene, glass mats, stainlesssteel meshes and glass fibers, both treated and untreated,have been nvestigated. Separation efficiencyofthese mediais affected by factors such as: fiber diameter, drop size, mediadensity, flowrate, depth of the media, its wettability and sur-face roughness. Researchers7.8 reported that a coalescermade of smaller fiber elements exhibits higher coalescenceefficiency. However, smaller fibers algo reduce the size of thedroplets leaving the bed. Therefore, using larger fibers inthe downstream endofthe media bed was recommended toincrease dropletsize leaving the bed which was then settledby gravity. Droplets released from the downstream end ofcoalescing media are typically in the 150 to 1,000 11m ange.9A common, but not universal theory, states that the dis-persed phase must preferentially wet at least one filamentofthe coalescence media. Consequently, when organics arethe dispersed phase, plastic filaments are frequently used.Selection of materiaIs as well as the fiber size and mediadensityare critical to the efTiciency and cost-effectivenessofthe coalescence.

A droplet must adhere to a filament long enough to bejoined by other droplets and grow in size (Fig. 3).10As theflow velocity through the media increases, there will comea point where the droplets are swept offthe filament beforethey have a chance to grow. The superficial velocity at whichthis happens is called tne critical velocity. 1'ypical com-mercial designs rangefrom 1 to 10 ft/min.Coalescer vessel geometry. The second tageof separa-tion occursafter he droplets havegrownand are released owhere they settle or rise by gravity. The time required foralI dispersedphasedroplets to forma secondhomogeneousphase s a function ofthe specificgravity differencebetweenthe phases, he continuous phase's viscosity, the size of

Gravity settlers. API's Manual an Dispa.S'a~f RefineryWastewater;6originally issued in 1969, gives the designmethad for sizing a liquid-liquid coalescerve$sel ased onHYDROCARBON PROCESSING I NOVEMRER 1991;

v = Ra e of rise of dispersed phase, crn/secI,' = Gravitational constant, 981 crn/sec21/ = Viscosity of water, poiseP", = Density ofwater, gm/ccPo = Density ofoil, grn/ccD = Diameter of oil droplets, cm.The manual assumes a worse case droplet size of 150~lm and gives the following:

Where V, = rate of rise of a 150 ~m oil globule in waste-water, ftJmin.With a knowledge of the maximum flow of oil entrain-ment in actual ft3!min., he size ofthe separator vesselcanhe calculated.


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exitthe coalescingmedia and therefore, permit calculationofthe vessel ize. In spite of all the investigative work daneto illuminate coalescing process, estidg is still requiredunder he specificprocess onditions o developa high eve~ \of confidence hat the required separation will occur n thi.Jplanto Testing can be dane in several different ways, eachproducing a different leveI ofknowledge and confidence:..Simple bottle testo A samp.leof the dispersion istaken, allowed to settle in a bottle and timed to the pointwhere the proper clarity is reached...Bench scale test through media. Small size andshort runs demonstrate the ability to break the emulsionand may give a first approximation of processconditions...Pilot plant testing. Larger diameter units (approx-imately 4 in.), and longer runs can producemore definitivedata, but changes during shipment and storage of emul-sion may introduce doubt about results...Slip stream testing at operational planto Testingunder actual processconditions can give the most reliabledata on which to scale-up o a full size unit (Fig. 8).

Molecular forces. The parameters that should be notedin testing with a chosencoalescingmedia are the specificgravity, interfacial tension, viscosity and concentration ofboth continuous and dispersed phases, velocity of flowthrough the media, temperature and of course,a measure-ment of the degree f separation.Suchmeasuremen~may bedoneby analysisofthe eflluents, photometric measurementof clarity, conductivity or other physical property. It wouldbe helpful to know the particle size distribution at the inletand on the downstream side of the coalescingelement, butsince suchmeasurements re very expensiveand time con,"~'...suming, direct measurementof particles is seldomdone. t.J)JCoalescence ftwo immiscible liquid systems s complexand yet critical to the successfuloperation of a refinery orpetrochemicalplanto There exists large databases ofbothpilot and commercial nstallations of porous media whichenhance he performanceoftraditional gravity separators.Whether he requirement sadditional capacity,higher sep-aration efficiency or environmental compliance,a knowl-

.M~dia f;;~ Looselykrntted lre mesh ;i~,i."'..~ Tightlyknittedwire meSh :!,~;;

:::;~'\C "'li,'";: Co-knitsofwire and nonmetalliclbe~;~;~"'"': ;:'~ suchasnylon,polypropylene, ,:i;ii;:fluorecarbons,lass ::cf~};~':"~'",\i",:~ Tightly nlttednonmetallic ompound\~!!..;

Compressed icro-fibersDfglass, \,;(c~ plasticsor stainless teel :j-"

Fig. 8. Portable lip-stream esting unit nstalled o collectactualpro-cess data.droplets onned and the characteristicsofthe thjn interfacialfIlmo The coalescer essel'sgeometrymust allow for time forthe droplets o reach he interface.Vesselsmust have a diarn-eter which wiil provide he required superficial !velocityanda length to the settling chamber which ailows for completeremoval ofthe dispersedphase rom the continupusphase. tmust algO rovide a place usually a boot) o control the nter-face and appropriate nstrumentation (Figs. 4 ,nd 5).In vertical-flow coalescers,he arrangement must ailowspace or the coalesced rops to settle (Fig. 6). !Whileverti-cal vesselsoffer a smaller footprint, the vessel~ay be argerin diameter than ones for horizontal flow beqause n set-tling,the droplets have nterference from the ~ounter-cur-rent flow ofthe continuous phase.Other methods to break emulsions. Ch~cals maybe used to precipitare emulsifying agents, o aff~ct he inter-racial tension, to neutralize electrical charges,j r to adjustthe pH. Electrical processes use a strong el,ctrical fieldto break someemulsions.While not a common~rocess, t isimportant in the desalting of crude oil. When the water inGil emulsion s passedbetween wo electrodes pd subjectedto high-potential pulsating current, the water globulesareattracted electricaily and coalesce owhere they can be separatedby gravity.While these methods are importantin specific cases, the most commonmethod of coalescences the physicalprocessof forcing the emulsion hrougha porousmedia.Table2 shows he mostcommonly used media for particularapplications.A specialcase s the three-phase ep-arator where a vapor is separated inaddition to the two liquid phases,ndus-try has used many arrangements forthis specialcaseand there is little liter-ature to provide a design basis, b\ltthree-phaseseparatorsare not uncom-mon in refineries as reflux separatorswhen distillation s aidedby stearn njec-tion. Fig. 7 showsan economicalype ofthree-phaseseparator.In whatever type or arrangement ofcoalescer, n initial calculation shouldbe basedonStokesLaw, o calculare hesettling time ofthe larger drops which

HYDR~CARBON PROCESSING I NOVEMBER 19958 '""_."',,---

Emulsion Problems

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