oily waste water cleanup by gas flotation

8/13/2019 Oily Waste Water Cleanup by Gas Flotation

1/17

West Indian Journal of Eogineering Vol 25. No . I. July 20 2) Technical Paper Moosai Dawe) 25 - 41

Oily Wastewater CleanupBy Gas FlotationA. Mooss'* A.A. Dawe

1. Introduction1 1 ily Wastewater

Produced wastewater from hydrocarbon reservoirs always containssom e oil. The fre e oil is often in an emulsifi ed form with the mediandroplet diameter usually in he range o f 3 -20 Wll . Gas fiotation o f oilywast ewater is a process in whi ch fine gas bubbles are inje cted into awater phase . Oil drpplets nd oil-coated solids , whi ch are suspendedin the water, be come aI/ached to th ese bubbles, rise to the surfac e, aretrapped in the resulting foam nd removed when the foam is skimmedfrom th e surface . Th e effectivenes s offlotatlon depends on the traditionalgravity separation parameters o f density dijJerence , oil droplet site ,

nd temperature but also critical are the gas bubble si t e distributionnd an even dispersion o f the bubbles. Gas flotation is particularlyvaluable for removing oil dropl e ts when tire oil density i s close to thatof water (i.e. , he avy oil). such as in Trinidad. This paper discusses theoperation o f h e gas fiotation pro cess for oily wastewat er including themechanisms behiT d th e proc ess , parti cularly the gas attachment to oildroplets . A cl earer understanding o f oiUbubble contact nd the influenceof sllrfactants in aiding this contact could make fiotation applicationsmore widely considered for Trinidad s o ily wastewater treatment .

Produced water is the water that comes with oil andgas during hydrocarbon production . t always containssome oil. The treatment of this produced oily water toan environmentally accepted level before disposal hasbecome mandatory in most countries . In Trinidad andTobago , government environmental pressure toimprov e the quality of oil - water separation in theoil fields is increasing, with 40 parts per miUion 40

mg per litre) being the quoted legislated standard .Elsewhere in environmentally sensitive countries. e .g.,Norway , lower figures are mentioned, even zero .This implies that dissolved and non-dissolvedcomponents will have to be removed from wastewaterbefore disposal.

1 Drops>ISO /U1t

in diameter , whichcan be separated by conventionalmethods , except for heavy oil discussedin 1.I below;

2. As free droplets 15 - ISO /U1t

3. As a stabilised often with indigenousanionic surfactants) oil-in-wateremulsion, with th median dropletdiameter usually in the range of

3 - 20 WlI [3].

Such oil-in-water emulsions are generated by shearingin pumps and by turbulent now in pipes as well asfrom steam injection EOR method:; . These crude -inwater emulsions are often stabilised by the variety ofsurfactants indigenous to the crude oil.he non-dissolved oil in wastewater is in three

forms as :

Petroleum Engineer, Petrotrin , Trinidad . ..mail: ro shnLmoosal @pr.tJ.Olrin .comPro e sso r, Pelroleum Engineering Uoil, Department of Cbemical Engineering , The Univel1ity o f the West IDdie , SI. Augustine , Trinidad .E-mail ; l ud w @ en/ij . uwLtt

Pertinent discussi on will be published in JanUAry . 2003 Wes t Indian Journal of i D e e r i n gif rece ived by Nov ember , 2002 .

25


2/17

West Indian JouroaI of Engineering Vol. 25. No . 1. (July 2(02) TecllnicaJ Paper Moosai Dawe) 25- 41

The formation of an emulsiou involves thecreation of enormous areas of interface. typically _ 1m'per cc of oil if the droplets are of 10 AlII in diameter .

The dissolved form has to be removed throughion exchange or bioremediation [1.2]; these processeswill not be discussed further here .

The separated water must be de-oiled beforedisposal. otherwise it could become an (e)(pensive)environmental embarrassment if oil pollutes potablewater sources or rivers . Large volumes of oilywastewater are being produced . Additionally. the wateris saline. with concentrations of salts as high as 25 by weight. but fortunately is usually less (rememberingthat seawater is ' 3 .5 ) and itself could be anenvironmental :'dzard.

The oily wastewater treating facilities currentlybeing used by many companies in Trinidad use onlygravity-induced separators and guard basins. however.discharge effluent often have oil concentrations thatexceed 50 parts per million . There are a wide varietyof separator designs and configurations. some withrather exotic proprietary internal devices .The equipment chosen for any wastewater treatmentfacility must be according to the characteristics of thewater to be treated and to its ultimate disposal. Thestandard oil water gravity separator is well discussed

elsewhere [4-7].

1 2 The Problems o eavy 11Heavy oil has a density close to that of water. Much ofTrinidad'soil is heavy so that due to this small densitydifferential, these oily wastewaters cannot be separatedinto oil and water by gravity separators . Add .itionally,these heavy crude oils tend to form stable emulsionsbecause of the waxes. asphaltines particles and otherimpurities present, which, in combination withproperties such as high viscosities and densities andfoaming characteristics, make these wastewatersdifficult to treat by traditional treatment systems .For heavy oil, the industry typically uses largeseparators . long retention times and heat plus dosingthe system with quantities of demulsifier chemicals .Such facilities are expensive and increase the cost of aproduct whose value is currently lower than that oflight crude . Additionally, the product may still not meetspecifications.

The small density difference between the heavyoil and water make the separation by hydrocyclone or

26

cenlrifuge ineffective [8]. Further problems are createdby the emulsions formed between the oil and the waterbeing extremely strong. However, gas flotation is ableto cope.

2. Gas FlotationGas flotation is a process in which numerousmicroscopic gas bubbles are injected into a water phasecontaining immiscible liquid (oil) or solid particles sothat the bubbles attach themselves to the particles anddecrease their density and enable separation . The oildroplets and oil -coated solids with attached gas rise tothe water surface, are trapped in the resulting foamand are removed when the foam is skimmed from thesurface . This foam is gently skimmed off while theclarified water flows out near the bottom of the tankFigures and 2).

In essence, the process of gas flotation is basedon gravity separation . By attaching gas bubbles todroplets of oil. the density difference between itselfand water is increased and the effective density of theoil is decreased and the oil appears lighter .Consequently, the oil rises faster, enabling faster anda more effective separation from the aqueous phase[9]. Clearly. as will be discussed later. the longer theresidence time of the gas bubbles in the notation tanks

(smaller bubbles). the greater the contact efficiency(the number of gas bubble-oil droplet collisions), andthus the greater the oil removal. Gas flotation canachieve the required standards by reducing theemulsified oil droplet concentration in mg per litre tosingle figures .

Although gas flotation has been used in thetreatment of oily wastewaters for man years [8], theprocesses happening within the gas flotation vesselhave somewhat limited descriptions in texts onwastewater . FurtherclarificatioD of he oil droplbubble

attachment process should enhance the selection andusage of types of chemicals, so as to optimise the entireprocess to achieve the desired environmentalrequirements at minimum cost This clarification iswhat this paper attempts to do .

2 1 Stokes LawThe governing priDciple of fluid separation is givenby Stokes Law . The velocity of bubble /drop rise in alarge volume of water has becn much studied [II, 12].Solution of the Navier Stokes equation for the terminalrise velocity, V for rigid spheres under the relevant


3/17

West Indian Journal of Engineering Vol. 25. No . I. (July 2(02) Technical Paper (Moo , ai Dawe) 5 - 41

mo t r

tas v ent

_ gas induction (mechanical or hydraulic)

+ ludge /grit

oily froth

,1

r gion

impeller J

FIGURE 1: Schemati c oj Ind uce d Gas F w ta ti on

gas venl Flo tali n ch amber

oily wastewateroily froth

gas - - 1 : - 1

wat er

I t ij;.


4/17

West Indian Journal of Engineering Vol. 25, No . I, July 2002) Technical Paper (Moosai Dawe) 25 - 41

conditions for flotation (laminar flow) gives StokesLaw ,

where

v _ d g(pw - P. )181tw

v = droplet settling (rising or faUing)velocity

d = droplet diameter= gravitational acceleration

w - P = difference in density betweencontinuous and droplet phase(oil or gas)

Ilw = dynamic viscosity ofcontinuous phase

Stokes Law hold s reasonably well for solids in therange 10


5/17

West Indian Journal of Engineering Vol. 25, No . I, (July 2002) TeclwicaJ Paper Moosai Dawe)25 - 41

An effective chemical programme(minimum quantities and maximumoil removal)

Aotation units have developed over tbe years,initially for mineral processing. The water travelsthrough the flotation chamber and by suitableplacement of bames and weirs moves downward stowards an exit, and the oily drops with attached gastends to move upwards . Good flotation for oilywastewater is brought about by the introduction of alot of fine bubbles . Tbe smaller tbe bubbles, the better,since tbe small bubbles can capture the small o ildroplets . However, once bubbles are released, they cancollide and coalesce into larger bubbles but largebubbles are ineffective for flotation. They rise rapidlycreating unfavourable turbulence and break-up ofbubble-droplet agglomerates . How the bubbles areintroduced into the water system is also important.Thus, critical design factors are the areas in the flotationchamber where the bubbles are created and where oiland gas attachment occurs .

Bubbles can be created in a number of ways,but in field practice, there are two common methodsof introducing the gas bubbles - those of induced gasflotation (sometimes termed dispersed gas flotation),lGF, and dissolved gas flotation, DOF Tbe significantdifferences between the two flotation processes are theaverage bubble size, the mixing conditions and thehydraulic loading rating, with induced flotation havingthe higher value. There are other more sophisticated(and expensive) methods such as electrolytic gasbubble generation creating stable bubbles of 1Of.U1ldiameter and gas sparging, where gas is pumpedthrough porous tubes to create the gas bubbles but theseare not discussed further here [Il).

3.1 Induced,r

DispersedGas Flotation lGFJIGF mechanically introduces the gas as bubbles .Initially, the gas is drawn into the mixing area, where(usually) a propeller mecbanically agitates the oilywater and shears the gas into - 100 1000 U1l diameterbubbles Figure 1). The gas is drawn into the waterand mixed with it , unlike DOF (discussed below) wheregas is dissolved in the water and comes out of solutionby depressurisation. The retention time in thewastewater treatment u .nit may be as low as four

minutes . Sludge is gradually formed from the input29

water and if this is disturbed by the propellers andmoved to the attachment zones, it can destroy theflotation process.

The IGFs units are usually multi-cell in designin order to improve their performance . The inlet gasnozzles, rotors or eductors (essentially vacuum pumps)and bames are patented designs . IGF units can beoperated at much higher hydraulic loading (higher flowthroughput for equivalent surface areas) than DOF.Hence , the capital costs for treatment equipment arelower than DOF but because of the large gas bubbleand small oil droplet sizes and the quiescent conditionsneeded for agglomerate formation,lGF is Dot now thepreferred option for oily wastewater cleanup .

3.2 Dissolved Gas Flotation DGF)DOF introduces the gas bubbles in a different way .Water is saturated with gas under pressure (up to fouratmospheres) , so that gas is dissolved in the water andis released from solution by pressure drop toatmospheric pressure in the flotation chamber . The gasevolves from solution as small bubbles , 100100f.Ulldiameter Figure 2) often described as malting thewater go milky white [II , 13]. DOF units are sodesigned that the gas bubbles form in the environmentof the oil droplets . Retention time in the flotationchamber is usually about 15 - 30 mins. and is a fairlygentle process . Suspended solids and minor entranceturbulence act as nuclei for bubble formation. Clearly,the amount of gas available for flotation is dependenton the operating pressure for the gas -saturating system.

The loading rates are smaller than IGF but thebubbles are smaller hence have longer retention timesin the flotation units, and slower particle rise rates.Multi- cell units are used in practice - often a set offive flotation cells . Turbulence is low in well-designedand properly operated systems . Chemicals , as

discussed later in 4 .4, are added to neutralise thenegative charges on the bubbles and drops in order toflocculate the oil , break the emulsion drops to formlarge flocs, so that bubbles have even greater oil-flocarea in which to attach . Gas bubbles are formed in andaround the suspended solids and emulsified oildroplets . However, the flocs can become a continuousoily sludge .

There are a number of ways of designing th .egas pressurisation systems, including [11]:


6/17

Wcst Indian Journal of Engineering Vol. 25. No . 1, (July 2002) Techni cal Paper (Moosai Da we) 25 - 41

Total pressar isa tion of all the wate rfeedwater Here , more gas lhr i s neededwhich should lead t o higher separa tionefficiency. Any so lids in the innuentmust be pumped through th epressurisation chamber to the flotati o nchamber whi ch ean des troy n oatingagglomerntes by co lli sion . Additionall y.

. th e oil in the wa stewater can befurth er emulsified within the pumps byshear . Higher power requirements areneeded t o run th e pumping sys tem s.

Pressarisalion of only parI of heinlet water C lea r y. lower power isrequired co mp ared t o the full strea mprcss uri satio n and less gas /hr .Some influent so lids and e mul sificat ionmust be pumped .

Recycle pressarisalion (Figure 2).Here. so me of the o utl et wat cr ispress uri sed with gas. up to about 60 psigand pa ssed back into th e n ota tionchambers . In the chamber. thepre ssure release generates Ihe bubblesfrom th e now s upe rsa turnt ed water .provided suitabl e nucl ea tion s itesare available . These are often cav itationturbul e nce . As the pr ess urised water isclean . no influent so lid s and emul s ionsare pumped thr ough the gas so luti onchamber with this pr ocess . The rec ycl ernti o of press urised water can be variedfrom zero to 100 . but typically 10-50 %.However. larg er notati on ce lls thanthe other method s are neede d (c reating

high er initial plant cost,). but it is nowthe prefe rred method .

3 3 GF esignsThere arc many djfferent designs of n otation un it withdesig n parnmeters spec ific t o manufacturer s pate nts .The des ig ns req uir e a wa y to introdu ce th e gas. amixing r egio n where the gas con tact s the oil droplet s,a flot ati on regio n where th e o il-ga s agglomeratesseparate which allows the m to ri se to s urfa ce and amean s to skim off the fr o th . C learl y. proper mixing of

water, chemicals and gas bubbles within the notati on30

unit mu st occ ur t o e nSure ma xi mum efficiency . Thisrequires proper equipment design . Ge nernlly , gas inputsrange from 0.2 to 05 scflbbl o f waler to be treatedwith water now r a tes o f 2-6 bbl s/hr per ft2of surfa cearea offlota tion ce ll. The ce ll depth s are 6-9 ft of water.For th e recycle press uri sation m etl...d . 10-5O of th etreated wat er is no nnall y recy cled . Fir stly. it is pumpedinto a sa turation chamber - a co lumn filled w ith ope nobjects to give a larg e s urfa ce area where th e gas isdisso lved und er press ure, usuall y 60 psig and thenpassed into the n ota tion ce ll thro ugh spec ial o rifice s.Unfo rtunat ely, chemical sca le ea n be fonned at theorifices from th e sa lts in th e water . Naturnl gas ca n bea safe ty ha zard if it is no t vent ed o r recycled carefully .

Ge nera ling small eno ugh bubbl es is c riti ca l butpresents pro blem s. One way is 10 add a small amountof a so lubl e s urfac tant (frother) . The emerging s treamof water , milky with mi cro -bubbl es. passes int o theflota tion chamber (F igure 2) . T he exact proc ess is notcl ea rl y und e r stoo d bUI Kitchener has given ade sc ripti o n [11]. In industrial prac ti ce , th e s up ersa tur ated water is fo rce d thr ough n eed le-va lves orspecia l or ifi ce s , a nd clouds of mi c ro bubbl es a rcpr odu ce d just downstream o f th e constriction .The mec hani sm by which the microbubbles are fonnedis that a certain minimum vel oc ity o f flo w through ano ri f ice is needed to pr o du ce a ny bubbles at all(otherwise th e water re m ains s up er-sa tu r a ted ) .

t see ms th at th e mi cro bubble s a re for med b y vapourpockets being tom off thi s cav ity, and they grow fromth e water st rea m as they are ca rried along .The micr ob ubbl es fo rm ed at a cavitating or ifi ceprob ab ly sta rt as s ubmicr osco pic vacuoles tom o tT theultr as o ni ca ll y osc ill ating vapour-cav.ity, fo r if thesuppl y press ure i s reduced j ust below that needed fo rvisible cavitation, virtually no microbubbles areproduced . A correc tly operati ng o rific e can be heard

emi tting a ' sizz ling ' so und . which is du e to cavitationand high -frequency osc ill atio n of a vapour cavi ty ju stbelow th e nec k. Neverthess. the o rifi ces used in OOFfor th e water injection also yie ld some gross ga sbubble s. prob abl y for med by growth of bubbles inco rners o r atta ched to the outlet pipe s. for here the wateris still partially super -sa turnted . T hese lar ge bubble scan rise rapidly and des lroy the qui esce nt co nditi onsneeded for flocc ulati on. Additionally . it has bee n foundth at when s up e rsat ur a te d water is added toagg lom ernted o il drops , bubble s can grow within the

agglomerates. suggesting that so me suitabl e nucl ei for


7/17

West Indian Journal of Engineering Vol. 25, No . I , (July 2002) Technical Paper (Moosai Dawe) 25 - 41

gas bubble generation are incorporated within theagglomerates .

Finally, the skimming device must remove thefroth layer containing the oil at the surface and notcreate disturbances which send oil back into the

flotation chamber .

3 4 Recent Developments InGas Flotation Design

Tbe designs ofDGF units, types of notation processesand chemical additives have undergone extensivemodifications t o improve effectiveness [13-23) .For instance, units include having multicell systemsand incorporating coalescing media , creating microflDcbubbles using a spinner design inside the vessel, havinga hydrocyclone in the inlet piping which removes solidsand larger drops of oi l before they enler the flotationzone, adjustable weirs which remove free oil enteringthe n otation section of the vessel and diverter barne sto effect some coalescence of drops [20] and evenincorporating a jet pump [21) . A recent notation columnsuggestion [22) is a multi-stage loop.Here, concentricdraft tubes establisb a multi-stage opera tion within thesingle column whicb cause in eacb stage, the gas boldup difference between the inside (riser) of the drafttube and the outside (downcomer) and induces a fluidcirculation which minimises back-mixing /entrainment.These are common problems affecting the effectivenessof convention .al flotation machines . Otber creativeadditions include using a spinner design inside Ihevessel or a hydrocyclone in the inlel piping [23] to aidthe collision and attachment probability between gasbubbles and oil particles .

4 Flotation Mechanisms

4.1 Flotation is achieved by enabling oil drop s toattach themselves to gas bubbles . This increase indensity differential makes tbe oil rise to the surface oftbe wastewater . The efficiency of separation can beincreased by coalescence of oil drops, which may beachieved by surfactants andlor chemical demulsificrs .Tbe attachment of oil drops to gas bubbles and Iheformation of a stable bubble drop aggregate are therate -co ntrolling steps. The mechanisms involved in tbeattachment of the gas bu bble to the oil dwp areessentially the same as coalescence between two oildrops, except that tbe gas bubble possesses elasticity;

hence tbe oil drop may bo unce off after the initial31

approach of the bubble and drop . Fortunalely, the dropdoes not always rebound, but may attach to tbe gasbubble and spread on its surface . Tbe period duringwhich the drop comes into contact with the bubble untilit is ready to bounce off, is called the induction time .

During this period , tbinning and rupture of theinterstitial water film mu st occur as described later .Ooly after this has been achieved will attachment occur .The crux of gas notation of oil is the adhesion of thegas bubble to the oil drop . Thi s c an only be optimisedin practice if the surface sc ience conditions are properlyunderstood so thai correct field conditions are created .

4 2 CoflisionTo begin with, the bubble and drop must fir st COmeinto clo se proximily. To ensure that this bappens , tbeirmutua l trajectories mu s t lead to a co llision stage(Figure . Hydrodynamic theories of the collisionbetween particle s and bubbles bave been developed,with that by Reay and Ratcliff generally accepted asbeing elegant and satisfactory (24).

Tbe collection efficiency, E, of a bubble and adrop i s the product of the collision efficiency, E, andthe attachmenl elficiency, E i.e ., E=E, *E, .

E,is the fraction of oil drops in the bubble s pathwhich collide with the gas bubble in the collision region(Figure . For parti cles >3 and


8/17


9/17


10/17

Wesllndian louma1 of Engineering Vol. 25, No . I, (July 2(02) Technical Paper (Moosai Dawe) 25 - 41

4 4 Chemical Additives Surfactantsnd Polyelectrolytes

The o il in wastewater is usually in the fonn of oi l inwater emulsion with most droplet diameter s being lessthan 20 Am with a median often around 51J111 . Thesetiny dr ops are pr evented from coa lesc ing by nega ti vecharg es o n the surface of the o il drop let s and gasbubble s due t o electrostatic forces created by tbeelectric -double layers . Tbese creat e a repulsive forcebetween the dr ops or bubbles which keep them apartand thu s hav e a low pr oba bilit y of coalescing .Separation by gravity by letting the was tewa ter sta nd(i .e., Stokes Law) can be dismissed as a demul sifyin gforce for indu stria l purpo ses, because the s mall size ofthe oi l drop s make this process extremely slow, e.g .estimate s [14] for a dens ity difference of O. lgm /ccpredi ct that a lOmm rise in an aqueous medium willtak e around 50 see for a 701Jl1l diam eter o il drop , 600sec for 20 1Jl1I and 3000 sec for 101JlII . Thus , beforenota tion ca n be effec tiv e industrially , the e mulsionmust be desta bilised and the o il droplets coa lesced .To break th e e mul sion, the accumulated electricalcharge on the bubble s and drops mu st be neutr alisedby introducing an opposite charge . This is achievedby demulsifier s. In t erms of phy sical interactions, thedemulsifi er serves

To modify the charg e on the o il dropl et

To cause n occ ulation b y anch orageand bridging mechani sms

Surfactants, or s urfa ce active agents , are moleeuleswhi ch are able to modif y tbe pr ope rties of an interface,c.g. , liquid /air or liquid /liquid by lowering the surfaceor interfacial tension . A surfa ctant p ossesses th efundamental characteristic of having two esse nti a lporti ons, one being water repellent , usually calledhydrophobi c (or oleo philic), the other being waterattractive, usually called hydrophili c (or oleopho bic).The hydroph ob ic portion comprises a co lle ction ofhydrocarhon groups , some at least of which f onn alinear chain which mayor may n ot be substi tut ed tovarying extents . The hydrophili c portion comprises asolubili s ing group such as sulphat e, sulph onate o rethoxylate.

The number and arrangement of the hydrocarbo ngroups toge ther with the nature and position of the

hydrophilic groups combine to d etermine the s urface4

active propertie s of the molec ule. If the hyd.rophilicportion is high molecular weight, it is tcnned a polymersurfa ctant. Surfactants fall int o four categoriesdepending on the distributi on of e lectrical charge onthe molecule viz:

Anionic in which the hydr o phobicportion of the m o lecule carries a

residual n ega tiv e charge, R COO- X+e.g., sod ium dod eey l SUlphate:

CHP- ,(C H,), H, OSO) - Na +

Cat ion.ic in whi ch the hydroph obicportion carries a residual positive

charge, R X - e.g., cc tyltrim et bylamm onium chloride:CH,(CI -12) 4CH,- N(C I-I., +C I-

No nionic ill whi ch there is noresidual electri cal charge , e.g.,dod ecy lalcobol ethoxy late:CH,(CHJIOCH,(OC H, H,) nOH

Amphoteric in whi ch both positiveand negative centres are to be

found in the moleeule, e.g .,alkyldimeth ylbetaine :

(CI-I.,)-,(CH,) -,R- N+ - C H, COO - .

These surfactan t molec ules o rient themse lves at theo il-water inte rface. A nocc ulant, which is a high weightcationic Or anionic polymer , is added after to crea temacronocs . Faster n otatio n results, according to StokesLaw, and a mOre eff icient se parati on. An emulsionbreaker is typically a high charg ed, low molec ularweight , ca tioni c polymer . They have a long chain andin addition to charge neutrali sa tion enables mechanicalbridging of oil drop s to c reate fl oes . An anionicpolymer , which has a high m olecular wei ght , can thenbe used to promote growth of the n oc th rough furth ermechani cal bridgin g. An eve n larg er rising veloc itywill occ ur according to Stokes Law when the radiu sof he 'o il drop increases throu gh such n oc fonnation .This will in tum increase the frequency of its co lli sionwith similar molecule s of o il , and thus fonn eve n largernocs . It will also increase the co llision rate with thegas bubble s in tbe wastewater notation tank, further


11/17

West Indian Journal of Engineering Vol. 25, No . I, (July 2(02) Technical Paper (Moo,ai Dawe 25 - 41

increasing the chances of oil /bubble attachment.Organic emulsion breakers, in addition to producingbetter effluent quality, ofte n require less dosage andproduce a smaller volume of s lud ge, than if aninorganic programme was employed .

Currently, in oily wastewater industrial practice,cationic and anionic surfa ctant and polymers are usedto tailor the noc Si7.e, noating characteristics and shearstrength . In practice, the chemicals u sed to giveoptimum res ul t s typically would be a primarycoagulant (emulsion breaker) being a low molecularweight, cationic (i.e ., positively charged) polymerwhich is abl e to neutralise the negative charge on theoil droplet as it adsorbs at the negatively charg edsurface of the oil droplet. This is followed by a highmolecular weight -2000) anionic polymer, often

derived from ethylene. propylene, vinyl, vinylidene orvinylacetate, which can then be used to promote growthof he noc through mechanical bridging . The preferredsize of the polymer molecule is such that severa l oilglobules can become attached to one polymer, whichcauses them to coalesce into larger particles by abridging mechanism .

4.5 Jar TestingThe maximum amount of surfactant that should be usedis that which creates the Critical Micelle Concentration ,cmc. at the salinity and tem perat ure of the wastewater[25J. The cmc is the point when a surface is coveredby a monolayer of the surfactant. At this point.aggregates of surfactant molecules (mice lles) begin toform in the bulk phase . Above this concentration, thesurfactant concentration is 'overshot' and causes thedroplets to gain increased charge and repel each otherso that the separation process lose s its effic iency. tcan cause th.e formation of foam between the su rfactantand gas bubble . This foam cannot efficiently trap oildrops and the entire purpose of surfactant usage willbe defeated. If too little surfactant is used. flotati onwill be less efficient since the conditions are notoptimum (perhaps 20 less effective). Hence, beforechemicals are added, their optimum concentration mustbe determined . This value will vary according to thefield cond ition s including diurnal temperaturevariations , concentration of wastewater. oil properties,and the salinity of water .

Unfo rtunately, information on nocculation andsurfactant composition used commercially is not oftendivulged in product literature, much to the disadvantage

35

of those wishing to minimise their cos ts or select theoptim um nocculants . To selec t and screen demulsifiers ,th e effect of various factors that affect oi l waterseparation need to be quantified , These include shear ,asphaltene content, water cut, demulsifier dosage and

mixing different crudes and temperature . Increasingthe temperature decreases the viscosity of the oil andemulsion , increases the frequency of th e dropco lli sions, decreasing the interfacial viscosity , causesa faster film drainage rate which in turn aidscoalescence greatly . Experiments must be carried outto identify suitabl e chemicals (s urfa cta nts) and theirconccntrdtions . The inlet wastewa ter stream must bemonitored closely , especially if surges in concentrationoccur frequently. Boule /ja r - testing or some ot her testis essentia l. Such tests can identify some chemicals ,

and eliminate othe rs, which might be effective underthe field conditions . Bottle tests involve the mixing ofthe various chemicals with samp les of the contamina tedwater , shaking and observi ng the results . It is thenormal procedure used in detcnnining the optimumconcentra tion of polymers needed for the flotationunit ' s treatment fluid at any point in time . It is arelatively quick method for selecting the chemical type ,dosages and treatment cond itions . t needs a smallvolume of material, a relatively short testing time andis relatively inexpensive. During full sca le opera tionjar-testing can be used to c heck c hemica l feed rdteswhich may lead t o a substantial decrease in the amountof chem icals used, the quantity of s ludg e produced, aswell as a bett er quality effl uent. However, a problemcan sometimes occur when sca ling -up the laborat orytest result s for application to field operatio ns, becausea surfactant ' s effectiveness is highly dependentparticularly on the cmc . which is affected by th e currentfield water c hara cteristics. The field conditions can bevery va.riable due to changes in production rates, streamsa lt concentrations , rainwat er additions andtemperature changes caused by weather or simp ly dayand night variations . Any deviations fro m the fieldconditions of even a well -plann ed laboratory jar-testingprogramme can be ruined by deviations of the fieldconditions for the labora tory tests, which then havedetrim ental effects on the notation efficiency.

4 6 Approach o f il Dropletnd Gas Bubble

As tw o drops (oil /oi l or oil /gas bubble) approach eachot her in a medium , there is a deformation of the drop s,


12/17

West Indian Jouma of Engineering Vol. 25, No. 1, (July 2002) Technical Paper (Moosai Dawe) 25 _41

4 6 Approach o f l l Dropletand Gas Bubble

As two drop s oilloil or oil/gas bubble) approach eachother in a medium, there is a deformation of the drops,which results in the formation of a dimple Figure5c) .

Thi s gradually disappears , giving way to a thininterstitial water film between the two oil dropletswhich must drain before it can rupture .

4 7 DrainageThe interstitial film drains under the combined actionof capillary suction and osmotic pressure, balanced bydisjoining pressure [26 -32]. Disjoining pressureconsists of van der Waals dispersion forces andelectrostatic forces . The capillary pressure is thepressure difference between the inside of the dropsbeing greater than the pressure outside. Gravity forcesplay only a minor role in the drainage process; theyslightly disturb the shape of the film, although thecharacteristic dimple is partly caused by the effects ofgrdvity . The dimple induces a pressure distribution inthe film. and hence fluid ./low. The lifetime of the filmis deterruined by the rate at which drainage takes place;it is the rate-limiting step of the coalescence process.The stability of this film i s a crucial factor indetermining the efficiency of the oil -water separationprocess .

The surfaces of the films are mobile and haveSUlfactants within the interfaces [26 -3 1]. The mainmovement within the fluid film occurs in the radialdirection with velocity varying with position in the filmFigure Sd) . The concentration of surfactants natural

or added) along the oil/water and gas /water surfacesat the centre faJls due to surface expansion caused bythe dimpling, creating a concentration gradient betweenthe centre region of the film and its surfaces . Thesurfactant concentration increases in the flow direction,which in tum causes a reduction in interfacial tensioncreating an interfacial tension gradient. This gradientalong the surfaces produces a force opposite to liquidflow and is known as a Gibbs -Marangoni clTcet [26 ,29] The movement of the surface from area s of lowinterfacial tension to the new areas of higher interfacialtension is acc o mpanied by a movement of bulk liquidwhich attempts to restore the film to its originalsurfactant concentrdtion . The monolayer of surfactantflows from regions of lower concentration to regionsof higher concentration 19wer interfacial tension)

36

creating a drag on the liquid adjacent t it in the thinlayer . The velocity of flow of monolayer material isgoverned by the viscosity of the interface and by tbebulk viscosity of the thin layer fluid.

Drainage therefore occurs because tbe surfaceof the film is mobile and the surface concentration atthe centre falls due to surface expansion . Drainageproceeds slowly and the film thins and the surfactantsredistribute themselves such that the concentration atthe centre gives an interfacial tension differenceFigure 5d) . The role interfacial tension plays in

drainage is tbus very important .The lifetime of the film is determined by the rate

at wbich drainage takes place , but all this bas to bappenin the short period a few milliseconds) whilst the gasbubble and oil drop are close together.

Tbe ideal surfactant to acbieve the speci ficpurpose of enhanced spreading is still uncertain (18].What is certain is that at surfactant concentrationsbelow or around the cmc, the adsorption of the surfaceactive molecules on the film surfaces and the propertiesof the adsorbed layers controlling the drainage andstability of the film are optimised [25 , 26].

The flow movement is illustrated in Figure Sd .The film, therefore, thins and when the criticalthickness is reached, ruptures.

4 8 OlllBubbieAttachmentthrough Rupture

When the liquid film reaches a thickness ofapproximately 0 .1 /lin ,disjoining pressure dominates .Here, very strong intermolecular forces come intoeffect, which lead to rupture of the film. Normally, thedisjoining pressure consists of the electrostaticrepulsive forces between the two overlapping surfacedouble layers, the Van der Waals forces among all themolecules of the film and the steric forces due to sterichindrance of closely packed molecules in monolayers[26-32 ].

_9 SpreadingImmediately following the rupture of the thin film, theoil must then spread over the gas bubble for flota tionto occur. F1uidl/luid interactions are usually describedby the spreading coefficients .The spreading coefficientof a fluid, S, is the imbalance between the interfacialtensions forces) acting along a single line contact linebetween fluid phases) [33]. For the gas -oil-water


13/17

West Indian J ournal of Engin eerin g Vo l. 25, N o. I, (July 2002) Tcchoical Paper Moosai Dawe) 25 - 41

sys tem, tbe o il spreading coe ffi cient On a water-ga sinterface is defined a s [34]:

So - Y - Yu - Yog

where Yw is tbe wate r-gas s urfa ce tension, Yow is tbe

o il-wate r int e rf ac ial tension and Y., is tbe o il- gassurface tension. S. needs to be positive f o r spre adingto occ ur: fo r o il-water systems , thi s is ge nerally tbecase .

Tb e s preading coe ffi cient i s a measure o f onefluid sprea din g s pont aneo us ly rel ative t o another on atbird pba se and therefo re as the va lue of S increases,tbe tende ncy to s pread in creases . For effec tive ga s

flotation, S. mu st be po sitive and indicate s tbar oiltend s to form a spreadin g co ntinuous film on the water

gas int erf ace becau se YW8 is la rge r th an th e s um o f

otber tw o interf ac ial tensions Y w + y E ~ a p e s of sprea ding and n on-s preading o ils

on gas are s bown in Figures and 7 [ 35 -38] . The sebeauti f ul pi c tu r es we re ta ke n fr o m a se ri es o fexpe rim ents pe rfo rm ed t o visu a lly o bse rve gas a nd o ildepr ess uri sati on beha vio ur in p oro us medi a us ingmi cro mod els and ar e highly relevant t o gas flotati on.T be sa nd gra ins a re the spots but d o not affec t th e gasoil sp read ing pbeno me na (o nly th e morpb ology o f tbebubbl es) so are not re levant to the di sc uss ion of the

n otati on of oi ly wastewat er. A po siti ve valu e o f S.means that whenever th e th ree pba ses o f gas , o il, andwat er co me int o co ntact , tb e o il pba se alwa ys fo rms acont inu o us film betw een th e gas a nd water (FiguresSa , 5b and ' ) . Sp rea ding of o il aro und th e gas e nsuresthe alla c hm ent of th e o il to tbe bubbl e is maintainedwhil e it rises to the s urfa ce . For a gas bubbl e of .50 /11

d,iamet er, a n o il drop with a di am eter of 20f.llll willfo rm a layer aro und tb e bubbl e o f around 1/11 andfo r a d ro p o f di amt er 10 /11 , a f ilm o f 0 .15f.1l11 .R oa ting o il dr o plets less th an 3 /1I is therefo re notgenerall y s uccess ful , as a very thin liru.1able film aroundth e bubb le wo uld be form ed, thu s, a noth er reaso n t ogrow th e o il dr oplets to aro und 20 l1Il by coa lesce ncebefo re all emptin g to float tbem .

Ifth e sys tem is non-oi l sprea ding , tb e adb ere nceoftb e o il to tbe ga s bubbl e is weak and the agglomerateis likely to brea k up as it rises ( Figure 7) .

37

f \

oily waste water cleanup by gas flotation

Documents