385_Modified Jet flotation in oil-water separation.pdf

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Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o l s u r f a

Modified jet flotation in oil (petroleum) emulsion/water separations

M. Santander 1, R.T. Rodrigues, J. Rubio ∗

Laboratório de Tecnologia Mineral e Ambiental, Departamento de Engenharia de Minas, PPG3M, Universidade Federal do Rio Grande do Sul (UFRGS),

Av. Bento Goncalves 9500, Prédio 75, Porto Alegre, RS 91501-970, Brazil

a r t i c l e i n f o

Article history:

Received 20 August 2010Received in revised form 2 December 2010

Accepted 10 December 2010Available online 23 December 2010

Keywords:

Jet flotationOily pollutantsPetroleum separationPlatform

a b s t r a c t

This work presents results of a rapid emulsified oil (petroleum) removal from water by flocculationfollowed by flotation in a modified jet (Jameson)cell (MJC).The modification is suchthat the downcomer

was sealedat the bottom (by a concentric blind-endtube) to allow floatedparticlesto enter immediatelyinto the frothy phase after the capture of the oily flocs by the bubbles. Also, a packed bed (crowder)was placed at the upper part of the concentric tube to stabilize the froth and facilitate the rise of the oilfloc/bubble aggregates. Thework wasdivided intotwo parts:a detailed laboratory study (1.3 m3/h)andapilot plant trial in an offshore platform. Parameters studied were flocculation (type and concentration of polymer), oil concentration, oil droplets size distribution and flotation cell design. Results of laboratorystudies showed mean separation efficiencies of the order of 80% when used as a conventional jet cell(CJC) with feed emulsions (droplets size of about 20m) ranged between 100 and 400mg/L petroleumconcentration. The oil removal increased up to 85% in the MJC. These studies allowed optimizing thedesign and process parameters: chemical, physico-chemical and operating. A MJC (5m 3/h) was thenprojected, built andinstalled in an offshore platform,afterthe oilextraction–productionpoint.At optimalconditions, in a single flotation stage, discharges varied between 20 and 30 mg/L oil concentration or81% removal at 24.7 m3/h m2 loading capacity. Because this jet cell operates with a high air hold-up, itpresented a very good efficiency (capture of oil droplets by bubbles) at low residence time (high-rateseparation) and showed to be simple, compact and easy to operate. It is believed that the MJC has agreat potential for treating polluted oily high flow wastewaters, at high separation rate. Results and

mechanisms involved are discussed in terms of interfacial phenomena and design factors.© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Mining, metallurgical, petroleum and chemical industries gen-erate huge amounts of wastewater usually polluted by solids,process chemicals, organic and other compounds [1–4]. Table 1summarizes main oil/organic sources reported.

During crude oil exploration and production large volumes of petroleum hydrocarbon bearing effluents, the so-called producedwaters, are concurrently recovered. These waters usually containhigh salinity, suspended solids (clay, sand, scale corrosion prod-

ucts), total dissolved solids and the oil may range between 100and1000 mg/L or still higher depending on oil separation process effi-ciency and nature of crude oil. Crude oils are a complex mixtureof many hydrocarbons which vary in their toxicity to aquatic andterrestrial life. These produced waters (after treatment) are more

∗ Corresponding author. Tel.: +55 51 33089479; fax: +55 51 33089477.E-mail address: [email protected] (J. Rubio).

URL: http://www.ufrgs.br/ltm (J. Rubio).1 Permanent address: Departamento de Metalurgia, CRIDESAT, Universidad de

Atacama, Av. Copayapu N◦ 485, Copiapó, Región de Atacama, Chile.

commonly discharged into the ocean environment and may causesevere environmental petroleum contamination especially whenreaching surface, ground and coastal waterways.

Therefore treatment of these effluents is required and mustresult in improved oil/water rapid separation, improved waterquality, oil recovery,water reuse,amongstothers. The conventionaltechnology for the treatment of oily produced water on offshoreplatforms usually includes a degasser (to remove the natural gasthat accompanies oil) and oil–water separators(mainly gravity set-tlers).

Many techniques for separation of oil–water emulsions areavailable, namely filters [5], ultra-filtration [6], micro-filtration [7],reverse osmosis, gravity separation, activated sludge treatment[7], dissolved air flotation [8,9], column flotation [10], flotationwith gas-aphrons [11], electroflotation [12], induced air flotation[13], membrane bioreactor [14], carbon adsorption, chemical coag-ulation, electrocoagulation. The advantages and disadvantages of these processes have been already fully discussed by Bande et al.[12].

The efficiency of these techniques depends on feed concentra-tion but mainly on the form of oil in the aqueous phase; if disperse(oil droplets>50m), emulsified (oil droplets < 50m) and dis-

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238 M. Santander et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244

Table 1

Main organic pollutants in different industrial activities.

Petroleum exploration and oilrefineries

Hydrocarbons, alcohols, ethers, phenols,carbon disulfide, sulfonic acids, etc.

Beneficiation of agates andamethysts

Minerals oils, dyes, diesel oil, etc.

Treatment of ores andmetallurgy

Organic solvents of solvent extractionprocesses flotation reagents: foaming,collectors and surface modifiers

Process metallurgy Cutting oils , solvents

Chemical and petrochemical Various oils and fats, organic reagents,surfactants

solved. The selection process for the treatment of oily producedwater oil platforms offshore will depend also on the equipmentfoot-print and performance because the space of the platform isreduced.

Yet, the flotation efficiency will be determined mainly by thedegree of emulsion destabilization stage. Poor performance hasalways beenobserved whenflocculation was incomplete.The flota-tion of organic rich waters such as oil spills on water, oily sewageor oil-in-water emulsions has been used for a number of decadesin various fields.

Because of collection and adhesion problems, the separation of the very fine oil droplets (<50m) by flotation requires fine bub-bles, quiescent hydrodynamic conditions in the cell and emulsionbreakers prior to flotation [15].

Table 2 summarizes most of the flotation processes applied tooily effluents including organic liquors and solvents.

1.1. The Jameson flotation cell – background

The cell (originally from the mineral processing field) consistsof an aeration/contact zone (the downcomer), a bubble-particleor aggregate disengagement zone (the tank proper pulp area)and a cleaning or froth forming zone (the tank proper zone). Incontrast to conventional mechanical flotation units, the Jameson

cell was designed to accomplish fast flotation based on a highbubble surface area flux, as a result of the very many fine bub-bles generated by high shear rate in the downcomer. The bubbles(medium size) formed in this cell may have 100–600 m in diam-eter [40,41].

In a Jameson cell, the mineral feed (or wastewater) and theair are introduced at the top and travel downwards in the down-comer which is essentially a vertical pipe. The pulp enters through

Table 2

Main studies reported on the oily water separation by flotation.

Authors Flotation system

Al-Shamrani et al. [9], Bennett [15], Bensadoket al. [16], Zouboulis and Avranas [17],Alkhatib and Thiem [18], R usso and Silveira[19], Belhateche [20]

DAF – dissolved airflotation

Welz et al. [13], Bennett [15], Belhateche [20],Angelidou et al. [21], Burkhardt et al. [22],Strickland [23], Burkhardt [24], Medrzyckaand Zwierzykowski [25,26], Medrzycka[27–30], Santander [31]

IAF – induced air flotation

Watcharasing et al. [11], Bennett [15],Santander [31], Gopalratnam et al. [32]

Nozzle flotation

Miller and Hupka [33], Beeby and Nicol [34],Lelinski [35], da Rosa and Rubio [36]

Centrifugal cyclone

Santander [31], Readett and Clayton [37] Jameson cellXiao-bing et al. [10], Gebhardt et al. [38],

Wyslouzil [39]Columns and DAF-columns

Bande et al. [12] ElectroflotationWatcharasing et al. [11] Flotation with gas aphrons

a feed line and forms a liquid jet. Air is entrained into the liq-uid in the downcomer by a vacuum effect and sheared intomany bubbles. Thus a very good environment for particle collec-tion by bubbles (air hold-up >40%) is created in the downcomer[42,43].

This cell has shown a great potential, nor only in mineral pro-cessing but also for solid/liquid separations and for liquid/liquidseparations [40,42]. Its main advantage is its high rate process effi-ciency and moderate equipment cost [41,44].

Problems with process efficiency have been recently solvedwith low-shear mixing head, recycle of treated water and use of polymeric flocculants in the downcomer. This allowed its use inwastewater treatment and recovery of solvent extraction liquors[37], municipal waters [45], treatment of wastes from a variety of industries, such as dairy factories, abattoirs, metal finishing, rollingmills, coke ovens.

Advantages recognized, amongst others, are:

• Compact design and low capital cost;• More, with no moving parts, the jet cell has low power con-

sumption and low maintenance costs: (air self-entrained, nocompressor or blower required);

•

Improvedperformanceinanumberofspecificareas:mineralpro-cessing, solvent extraction,industrial(treatmentof effluents) andmunicipal wastewater industries;

• Low residence times (<3min), high throughput and high effi-ciency.

In the CJC modification to the basic design [46], for effluentscontaining fragile flocs, the feed containing suspended particu-lates is mixed with a bubbly flow generated by a plunging jetof recycled clean liquid, in a relatively quiescent zone in thedowncomer.

The flotation separation of very fine oil droplets (2–30 m) iseven more complicated and usually requires fine bubbles, qui-escent hydrodynamic conditions in the cell separation zone or

emulsion breakers prior to flotation [32]. This is due to collectionand adhesion factors, which makes the process very slow, espe-ciallywhen, treating high flow-rates. IAF(induced airflotation)andDAF, have beenused extensively in the removal of stable oily emul-sions [15,20,23]. IAF utilizes bubbles between 40 and 1000m insize and turbulent hydrodynamic conditions. The process has lowretention times, normally <5 min. Conversely, DAF employs micro-bubbles (30–100m), and quiescent regimes. However, becauseretention times are higher (20–60min), this process is inefficientwhen treating voluminous effluents at high flow-rates and highloadings capacities (>7m3/h m2).

The Jameson cell, column flotation with colloidal gas aphrons(CGA) (pre-reagentized gas bubbles) and conventional columns arenow being utilized in solvent extraction plants [37]. Here the flota-

tion devices are used in the discharge aqueous streams from theSX-EW (solvent extraction-electrowinning) plant to recover theorganic liquor lost by entrainment into the aqueous phase. Thus,flotation can reduce organic losses and reduce potential environ-mental problems.

Amodifiedjetflotationcell(MJC)hasbeendesignedinourlabtoaccount for a better oil droplet capture by bubbles, for the decreasein the amount of “short circuit” observed in the conventional unit,all at high loading treatment capacity, the latter being extremelyimportant in off-shore platforms [47]. This work presents results of oil (petroleum)/water separation at laboratory (continuous work)and pilot plant, comparing process efficiencies between the MJCwith the CJC (the conventional jet cell unit) in a small continuouslaboratory plant and operating, withthe MJC, in a Brazilianoffshoreplatform.



M. Santander et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244 239

Fig. 1. (a) Conventional jet flotation cell, CJC; and (b) modified jet cell, MJC.

2. Experimental

2.1. Bench studies

2.1.1. Materials

2.1.1.1. Petroleum emulsions. These were prepared to simulate off-shore petroleum effluents, in a salty medium (75 g/L NaCl) using aheavy oil, crude petroleum. Main physicochemical characteristics

of the oil were: real density of 0.890 kg/m3 (at 30◦

C); a viscosity of 0.042Ns/m2, surface tension 34.4 mN/m.

The oily waters (oil/water emulsions) were generated in a con-tinuous high-pressure homogenizer system [31]. The oil/watermixtures were pumped, with a helical pump of high pressure(25kgf/cm2) working at 2m3/h capacity, containing varying con-centrations of oil and NaCl (0–75 g/L), through stainless steelhomogenizer plates with holes of different diameters (3–7 mm).Thetwo-phase mixture issubjectedto intenseturbulenceand shearby the conversion of pressure to kinetic energy, thereby leading tobreakup of the dispersed phase into small oil droplets, producingstable emulsions with different size distributions. Oily waterswerecharacterized with respect to size distribution of the oil dropletsusingan online particlesize analyzer(MalvernmodelSystem 3601)

and concentration of oil (petroleum) by UV spectroscopy. Samplesforcharacterization were collected with theaid of samplers locatedin the tube carrying the oily water feeding the flotation cell. Sam-ples were collected every hour and results are the mean of about 6aliquots per day.

2.1.1.2. Flocculant. A non-ionic flocculant polymer, polyvinyl alco-hol (PVA) to destabilize the oil-in-water emulsions.

2.1.1.3. Frothers. SodiumDodecylSulfate(SDS)andDowfroth1012(DF-1012), a methyl-polypropylene glycol, were employed to assistboth, the air entrainment in the downcomer and the stabilizationof the froth for keeping the floated oily flocs.

2.1.2. Flotation studiesThe separation of oil from oil/water emulsions were conducted

in the CJC and results compared to those obtained in the MJC asdepicted in Fig. 1(a) and (b), respectively.

Both lab flotation cells had 1.3 m3/h capacities and consisted of a downcomer of the jet, a flotation tank and a level control system.The downcomer had 0.05m diameter, 2 m height, immersed 0.1minside the concentric collector. The nozzle (6 mm diameter) had an

air inlet orifice of 5mm diameter. The cell tank (PVC) was 0.25mdiameter, 0.71m height (35L capacity) and the froth launder, 0.4 mlength×0.4m width×0.2 m height. Further details are shown inTable 3.

In both cells the “capture” stage (collision–adhesion) to form oilfloc/bubble aggregates occurs in the downcomer which are sepa-rated off from the water in the tank (phase separation) [42]. Theoily water was conditioned in a static mixer, with the PVA floccu-lant and fed to the jet cell. The reduction of pressure at the inletinduced air intake which results in dispersed small bubbles. Theseare forced to descend with the liquid counter to their buoyancy inthe direction of the flotation tank.

In the MJC unit, a CJC has been redesigned placing an inter-nal cylinder (blind-end) which receive the downcomer suspension

allowing allparticlesto enterimmediatelyin theseparationzone bythetop(Figs.1and2). Also, a packed bed (crowder)has been placedat the flotation cell separator tank to stabilize the froth and facili-tate the oil/bubble aggregates rising up, without much turbulenceto break them apart.

Table 3

Jet flotation cells: CJC or MJC main operating and design parameters.

Flow rate 1.3 m3/hRetention time of downcomer 10.9 sRetention time in the flotation cell 96.9 sDiameter of downcomer 50.0 mmHeight of downcomer 2.0 mDiameter nozzle constriction 6.0 mmFlotation cell volume 0.035 m3




Fig.2. Modifiedjetflotation.Detailsofthecylinder(blind)atthebottomandpackedbed (crowder) at the top.

Flotation studies as a function of droplets size (volumetric size,d(4,3)) and oil concentration, were conducted in the CJC with3 L/min air and 20 L/min oil feed flowing through an static mixer(as a flash flocculator), 3 mg/L PVA and 28mg/L DSS [47]. Flotationruns lasted 6 h and samples (100mL), of the treated water, werecollected (for oil analysis), hourly in aliquots taken within 2–3min.

Results are mean values of triplicates tests and the standard errorof the mean was about ±5% for a 95% confidence interval. Somefurther operating parameters and cells design characteristics aresummarized in Table 3.

2.2. Pilot plant studies in an oil field platform

Studies were performed in a maritime platform, named PNA-

1 (from Petrobras, Rio de Janeiro, Brazil), in the MJC withpetroleum/water emulsions from two different points of collection(Fig.3). Thestudied parameters were the type of aspirated gas(nat-ural gas or air), concentration of oil and type and concentration of flocculant(PVAandDismulgan,acationicpolyacrylamide),butonlyoptimized conditions (long-time experiments) are presented. Dis-tribution size drops in emulsions wasnot measured, but areknownto be similarto those studied at laboratory scale. Samples were col-lected every hour and results are the mean of about 6 aliquots perday. Results are mean values of triplicates tests and the standarderror of the mean was about ±5% for a 95% confidence interval.

Bench studied allowed to design and conduct studies of MJCflotation of the oily water effluent from the degassing vessel. Thisoily water was pumped at a 5.0 m3/h flow rate and fed to the MJCusing an helical pump of 8.3m3/h capacity. The flocculant solutionwas injected into the helical pump discharge pipe endowed with astatic mixer (800 mm long and 25 mm in diameter).

The MJC downcomer was 0.1 m diameter, 2 m height and wasimmersed 0.7m inside the concentric collector. The residence time

Fig. 3. Oil field extraction and oily waters treatment systems. PNA-1-oil field platform from Petrobras, Rio de Janeiro, Brazil.




10 20 30 40 500

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

d(4,3) (mm)

R

( % )

[ O i l ] f ( m g / L )

Fig. 4. Flotationof emulsifiedoil (residualoil)in theCJC as a function ofoil dropletssize(volumetricdiameter). [Oil]initial = 90–120 mg/L,[PVA]= 3mg/L,[DSS] =27 mg/L,3 L/min air flow rate, temperature 27–35 ◦C and pH 6.5.

of the slurry was 6.8 s for an estimated 40% holdup. The nozzle(12.25 mm diameter) had an air inlet orifice of 5 mm diameter.The cell tank was 0.5 m diameter, 1 m height (196L capacity and2.36 min residence time) and the froth launder, 0.74m diameterand 0.15–0.25 m deep. The blind bottom had 0.3 m diameter and0.75 m height. All pieces were built using 316 stainless steel.

3. Results and discussion

3.1. Laboratory studies

3.1.1. Conventional jet cell – CJC Fig. 4 shows that if d(4,3) decreases from 40 to 10m the

removal percentage decreases from about 90% to 70%, increasingtheresidualoilconcentrationfrom10to30mg/L.If d(4,3)decreasesfrom 40 to 12m, the concentration of droplets <10m increasesfrom zero to 37% (see Fig. 5). Thus, according to expectations the

collection efficiencyof droplets by bubblesdecreases with decreas-ingtheir size as in flotation of mineral particles.This is because theyfollow the streamlines around the bubble and not collide with thebubble surface which in this CJC (the bubbles) may be as big as0.1–0.8mm [40,41].

Strickland [23] and Sato et al. [48] reported that oil recoveryenhances while oil droplets size increases and bubbles decreases.Satoexplainsthatthisisduetoatleast,twoeffects,thefirstgeomet-ric and a second thermodynamics. Accordingly, small bubbleshave

1 10 1000

20

40

60

80

100

Droplet diameter (µm)

C u m u l a t i v e f r e q u e n c y (

% )

0

5

10

15

20

P a r t i a l f r e q u e n c y ( % )

d(4,3) = 12 mm

d(4,3) = 40 mm

Fig. 5. Comparison of droplet size distribution in the oily waters with different

volumetric mean diameters.

100 10000

80

160

240

320

400

[Oil]i (mg/L)

[ O i l ] f

( m g / L )

0

20

40

60

80

100

R ( % )

Fig. 6. Flotation of emulsified oil in the CJC. Effect of feed (initial) oil concentrationon oil/water separation efficiency. [PVA]= 3 mg/L, [DSS]= 27mg/L, 3 L/min air flowrate, temperature 27–35 ◦C, d(4,3)=23m and pH 6.5.

high projected area which facilitates collisions with small dropletstogether with a suitable hydrodynamics. Yet, Pal and Masliyah [49]

found, in studies of removal of chemically emulsified oils (usingnon-ionic surfactants), that a decrease in the size of the bubblesdecreased the efficiency of separation under strong stirring (hencethe degree of mixing) within the collection zone of the flotationcolumn.

An increase in the degree of mixing causes a reduction in thecapture efficiency despite the increase in the frequency of colli-sions (due to the bubbles fineness). It is known that oil drops mustdisplace the thin liquidfilm that surrounds droplets andair bubblesand this does not occur if the time required for the film thinning islonger than the time of contact; in this case (with strong stirring),the time decreases with the increase in the degree of mixing.

Fig. 6 shows that, up to approximately 400 mg/L, the removalremains approximately constant at 80%. For concentrations higher

than 400mg/L, this percentage decreasesdramatically down to 60%oil removal. These values were obtained under optimized condi-tions involved various parameters [31]. The optimal parametersfound and reported in Santander [31] were: flow-rate: 20 L/h oilywater conditioned in a static mixer with 3mg/L PVA, 28mg/L deSDS and 3 L/min air flow-rate.

The decrease in the oil separation efficiency in the CJC appearsto be due to, at least, the following phenomena: (a) Low oil feedconcentration; (b) increase of the proportion of droplets <10mgenerating the problems already discussed; and (c) hydrodynamicturbulence in the flotation device at concentration >400mg/L.

Also, in this CJC the instability of flow in the separation tankgeneratesturbulence because thejet entersdownwards with a highspeed, increasing the drag of the finest oildroplets, especially those

either not flocculated or weakly attached to the air bubbles.Bubbles play the vital role of actually separating the solids (or

droplets in emulsions) from theliquid phase which ultimatelyleadsto their removal. It has been well demonstrated that the bubblesize is one of the most important physical variables determiningflotation efficiency [50–54]. Bennett et al. [55] were the first toreportthe effect of bubblesize andfoundthat smaller bubblesweremore efficient in the flotation of coal. As Ahmed and Jameson [52]and Yoon [53] suggested, the use of small bubbles (in the rangeof 100–400m), and a more quiescent environment than the agi-tated cells, it will improved the recovery rates associated with fineparticles.

To date there is agreement that a decrease in the bubble sizewill improve oil droplet recovery because of increased surface area

and, therefore, greater probability of contact between bubbles and




Table 4

Effect of the average diameter of oil droplets on the MJC oil/water separation effi-ciency. Air flow rate, 5–9L/min (maximum self-aspirated air).

d(4,3) (m) [Oil]initial (mg/L) [Oil]final (mg/L) R (%)

23.96 98.1 16.9 8311.77 91.5 16.0 8224.43 161.7 25.6 8412.15 161.2 21.8 86

particles. Nevertheless, if the bubbles are too small of course, theywill not be able to providesufficient buoyancy to lift large particles,but this is not usually a limitation observed with oil droplets. Forthesame overall air flow rate,the number ofbubbles in the suspen-sion, andhence the surface area availablefor contact with particles,increases as the bubble size decreases. The bubble size obviouslyaffects the bubble surface area flux and, hence, the carrying capac-ity of the device. It has also an effect on the capture efficiency and,consequently, the kinetic rate constant, which is often dependenton the ratio of the particle diameter to the bubble diameter, and tothe bubble diameter itself.

Gas holdup (which controls the bubble surface area flux) in thedowncomer may reach as high as 50–70%, depending on the design

and operation conditions [44]. Many design and operating vari-ables affect gas holdup [44,56]. In recent studies involving the CJCor Jameson cell [44], the effect of several parameters, such as thenozzle diameter, the downcomer diameter, the free jet length, the

jet velocity and the ratio of air–feed flow rate on holdup and airentrainment rate were reported. Holdup increased with increas-ing downcomer diameter, jet velocity and length and air/feed flowrate ratio but with decreasing nozzle diameter. Higher holdup val-ues were obtained if a small size nozzle diameter is used with largesize downcomer diameter.

3.1.2. Modified jet flotation cell (MJC)The test conditions were equal to those used in the CJC, except

for the self-aspirated air flow which was found to be different.

Table 4 shows that, with average oil droplets of the order 12m(indiameter), was possible to reach removal efficiencies higher than82%. This table also shows that removal efficiency is higher for thehighest oil initial concentration. Removal (separation) efficienciesof about 82% were attained with90 mg/L (initial) oilconcentrationsand 86% with 160 mg/L. Results might be explained in terms of thedegree of turbulence in the downcomer, which is a function of theamountof airaspirated andappearsto dependon theconcentrationof oil. Here (in separated tests), it was found that for 600 mg/L of oil, the maximum airflow aspirated was 5 L/min and with 100 mg/L of oil, this increases to 10L/min. Again, the higher degree of turbu-lence will make that the jet enters downwards witha higher speed,increasing the drag of the finest oil droplets, especially those eithernot flocculated or weakly attached to the air bubbles, decreasing

the overall oil removal.Fig. 7 compares the separation performance of CJC and MJC,

as a function of the initial concentration of oil, between 100 and600mg/L. The increase in the efficiency of separation of the oildroplets with respect to theconventional jetcell,withsmaller sizes,again confirms a better capture of the oil flocs together with anefficient oily flocs/water separation at the flotation tank.

Comparatively, the short circuit (oil droplets reported to thetreated water, in this case) commonly observed in this type of jetcell is highly reduced by the concentric blind cylinder receiving thedowncomer flux [31,57]. Thus, under the same experimental con-ditions, the modified cell presented (always) removal efficiencies,at least, 5% higher than the conventional. The concentric cilindricalcollector, in the MJC, avoid the carry-over of droplets of oil not fully

adhered to the air bubbles. Because loaded bubbles ascend, a new

100 10000

80

160

240

320

400

0

20

40

60

80

100

R ( % )

CJCMJC

[Oil]i (mg/L)

[ O i l ] f

( m g / L )

Fig. 7. Comparison of oil/water flotation separation efficiencies between MJC andCJC, as a function of the oil initial concentration. Flotation feed = 1.3 m3/h, air flowrate ranging from 4 to 9L/min (maximum self-aspirated), in MJC and 3L/min in theCJC.

Table 5

Studies of removal of oil from oily water (effluent from the degassing vessel) by

flotation in the MJC. Feed rate, 5 m3

/h.

Flocculant flow rate, mL/min 100Concentration of PVA, mg/L 3Concentration of Dismulgan, mg/L 10Pressure, power input to the jet cell, kgf/cm2 0.45[Oil] Feed, mg/L 68.7[Oil] Discharge, mg/L 29.7Separation efficiency, % 56.8

re-flotation mayoccurinside theblind tube. This avoid or decreasesthe observed short-circuit in the conventional cell. Further, the factthat the discharge of the jet is performed in this cylinder bearinga packed bed, stabilize the flow and the froth allowing a minimumfloc breakage.

3.2. Flotation studies with the MJC in an offshore platform

The studies were performed with samples collected in two dif-ferent points (see Fig. 3).

3.2.1. Oil/water separation in the effluent from the degassing vessel

Table 5 and Fig.8 showthe results of studies of oilremoval usingoily water from the degassing vessel (see Fig. 3). Results show that

Fig. 8. Separation of emulsified oil in water (degassing vessel) by flotation in theMJC (loading capacity of 24.7m/h). Flotation feed, 5m3/h, 10mg/L of Dismulgan

and 3 mg/L of PVA.




Table 6

Results of the separation of oil from the oily water, effluent of the separator pro-duction, by flotation in theMJC.10 mg/L of Dismulganand 3mg/L of PVA. Feed rate,5 m3/h.

Flocculant flow rate, mL/min 100Concentration of PVA, mg/L 3Concentration of Dismulgan, mg/L 10Pressure, power input to the jet cell, kgf/cm2 1.15[Oil] Feed, mg/L 123[Oil] Discharge, mg/L 30

Separation efficiency, % 76

theremovalefficienciesweresmallerthanthoseobtainedwithsyn-thetic oily waters under similar conditions, namely temperature,salinity and concentration of oil. It is believed that this low effi-ciency was due to the presence of (a) residual chemicals such asdemulsifiers, and corrosion inhibitors that reduce the action of thePVA flocculant or (b) colloidal solids,coated with oil, which are notbeing flocculated by PVA. It wasvisuallyobserveda high proportionof treated oily water with dispersed colloidalsolids(even a sludge),especially with the flocculant Dismulgan.

3.2.2. Oil/water separation in the effluent from the production

separator In Table 6 and Fig. 9 are described the operating parameters and

the results of oil flotation in the MJC with the oily water from theproduction separator. In these tests the MJC operated with auto-aspirated natural gas from the degassing tank. Results showed thatremoval efficiency is higher than those obtained with oily watersfrom the degassing vessel. This removal efficiency is probably dueto the fact that in this case, the oil was less emulsified (largerdroplet size) and that the airflow aspirated was lower (less tur-bulence inside the downcomer) and also no formation of sludgewas observed. The results obtained are within Brazilian emissionstandards of oil discharged to the sea (20–30 mg/L).

The modified jet cell presented a very good performance intreating a typical Brazilian offshore oil bearing effluent confirm-ing its potential validating the bench results. In a single run (onestage only was sufficient) the oil content was always below stan-dard limits. Main advantages observed were: the compact design,low residence times (<3min), high throughput and high efficiency,at long times. It is believed that this process might be useful intreating emulsified petroleum or oil spills at high rate, in platformand superficial waters.

Fig. 9. Separation of emulsified oil in water (production separator) by flotation intheMJC (loadingcapacityof 24.7m/h). Flotationfeed, 5m3/h,10 mg/L of Dismulgan

and 3mg/L of PVA.

4. Conclusions

The removal of oil (petroleum) from oily water at concentra-tions ranging from 50 to 600 mg/L and volume mean diameter of oil droplets in the order of 20m is efficient with both types of

jet flotation cell, the conventional CJC, or the modified, MJC. Inthis new design, the slurry enters directly (through a concentricblind-end tube) into the frothy phase after the capture of the oilyflocs by the bubbles. Because loaded bubbles ascend, a new re-flotation may occur inside the blind tube. This avoid or decreasesthe observed short-circuit in the conventional cell. Also, a crowder(packed bed) was included, at the top of the concentric blind-endtube, to stabilize the flow and the floated oily flocs. Results showthat this cell is more accurate than the conventional cell yieldinghigh oil removal values and treated water with low oil levels. TheMCJ design resulted in an increase in theoil removal efficiency by asteady 5% percentage points (from 80%in the CJC to 85%in the MJC)in long operations. This increase in separation efficiency is due tothedecreaseindragofsmalldropletsdonotadheretoairbubblesasthe new mode of operation downcomer; hydrodynamic conditionsof high turbulence, promotes flocculation (coalescence) of oil. Thetesting of the MJC (5 m3/h) in a maritime platform demonstratedthe efficiency of this rapid flotation device, obtaining high values of

oil removal at a high throughput rate, reaching the allowable levelsfor discharge of oil in one single step. These values ranged between20 and 30 mg/L of oil, with removal efficiencies of around 81%, at24.7m3/h m2 throughput. The MJC shows simplicity in design andhas great potential for the treatment of oily wastewaters at highrates.

Acknowledgements

The authors are grateful to FINEP, CNPq, CAPES, UFRGS andPETROBRAS(all Brazilian Agencies) for their financial support. Spe-cial thanks to O. de Aquino (Petrobras), S. Amaral, J.J. Rosa (UFRGS)and B. Zazzali (UDA-Chile), for their technical assistance.

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