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Separation of oil-water mixturein tankS. K. Das a & M. N. Biswas aa Chemical Engineering Department , Indian Instituteof Technology , Kharagpur, IndiaPublished online: 09 Sep 2010.

To cite this article: S. K. Das & M. N. Biswas (2003) Separation of oil-watermixture in tank, Chemical Engineering Communications, 190:1, 116-127, DOI:10.1080/00986440302095

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SEPARATIONOFOIL-WATERMIXTURE IN TANK

S. K. DAS

Chemical Engineering Department,Indian Institute ofTechnology, Kharagpur, India

M. N. BISWAS

Chemical Engineering Department,Indian Institute of Technology, Kharagpur, India

Experimental investigations have been carried out to separate diesel oil-water

mixture in unbaffled, baffled (with different inclination of baffle), and variable

area baffled separators. A baffle separator with a 45� inclination to vertical

has been found to be most suitable for the oil-water separation.

Keywords: Oil-water separation; Baffled separator; Variable area baffle

separator

INTRODUCTION

Accidental spillage of oil results in a severe loss of revenue and persistentenvironmental pollution. To meet stringent environmental protectionregulations, it is imperative to process a large amount of oily water,normally at high flow rate and low oil concentration. Oil-contaminatedwater in the offshore situation enters the environment from two mainsources, i.e., produced water and platform drainage. The consent todischarge generally requires no visible oil in the effluent stream. On landthis effectively means 2�10 mg=L of separable oil residue; offshore a levelup to 40 mg=L can be tolerated.

Received 2 November 2000; in final form 27 July 2001.

Address correspondence to S. K. Das, Chemical Engineering Department, University

of Calcutta, 92, A. P. C. Road, Kolkata 700 009, India. E-mail: sudipcuce@hotmail.com

Chem. Eng. Comm.,190: 116�127, 2003Copyright# 2003 Taylor & Francis

0098-6445/03 $12.00+ .00

DOI: 10.1080/00986440390171062

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The conventional API (American Petroleum Institute) separatorconsists of a channel or channels through which water passes horizontallyat a velocity that allows the oil droplets sufficient time to rise to thesurface, where they are skimmed off. The standard API separator wasdesigned to achieve the separation of oil globules larger than 150 mm indiameter. In practice this means that the effluent quality will not be betterthan 50 mg=L of separable oil because a sufficient proportion of the oilpresent is below 150 mm in diameter. Since the rising velocity of the oildroplets governs the physical separation of oil from water, which followsStokes’ law, the separation of smaller oil droplets will require an inor-dinately larger volume of API separator. The plate separator is the mostwidely used of the plate systems. It is designed to capture 60 mm oildroplets. A multibed or multimedia filter bed containing granular bedsof sand, gravel, or a mixture of different materials can be used as apretreatment step to limit the oil concentration in the feed. Furthermore,chemical flocculation can also be used to assist separation. The filter bedis used as a physical barrier on which the oil droplets can coalesce. Onbackwashing the filter bed at the end of the run the oil is removed withthe backwash and can be separated easily. This is thus a cyclic operationsimilar to the filter operation. The use of air- or gas-assisted flotation forthe separation of oil from water is also practiced industrially. Themicroscopic bubbles attach to the oil droplets so that their effectivespecific gravity reduces and hence increases their raising velocity. How-ever, this involves higher operating cost than the gravity separator.Coalescers are basically improved filtration systems using media to pro-vide a surface on which the oil droplets will be retained and will coalesceinto large droplets for easy separation. Coalescers using a DC electricfield can achieve a continuous separation of oil and water. The membraneprocess is based on the sieving action of a polymeric material controllingthe flow of molecules larger than the membrane pores. Membraneseparation basically operates under high pressure to force water throughthe membrane with retention of all other contaminants. Microbialtreatment must typically be used with less than 10 ppm of oil (Ouelletteand Cheremisinoff, 1985).

Delaine (1985) discussed the characteristics of the offshore effluentstream, the different types of separators, and their advantages anddisadvantages. It has been shown that separation basically depends onthe concentration and drop size distribution of the dispersed phasein the liquid-liquid emulsions. Shiragami et al. (1988) investigatedthe enhancement of batch settling by inclined plates. Hoshino et al.(1984) confirmed that a glass fiber filter was much superior to cellulose forfiltration of oil-water emulsion. Sato et al. (1980) attempted to separatedilute emulsified particles of heavy oil by means of air flotation. VanHam et al. (1983) studied air flotation of a highly stabilized oil-in-water

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emulsion in a batch process. They observed that the performance wasstrongly affected by the method of air distributor (porous plate, singlehole, or multiple hole plate), initial oil concentration (30�600 mg=L), oildrop size distribution in the feed, surfactant type, and gas superficialvelocity. Bails and Larkai (1982) and Joos and Snadden (1985) studiedthe performance of electrostatic coalescers for different oil-water systems.Hsu (1986) observed that more effective separations have been achievedby centrifugation at the expense of very high operating cost. Unno et al.(1986) used PTFE membrane for oil-water separation. Delaine (1985)gives the comparative data for different separation processes. The plateseparator appears to be the most widely used separator system, parti-cularly in offshore operations, due to its low operating cost. But thequality of effluent stream is comparatively poor. Stringent environmentalregulations sometimes inhibit its use. The present paper deals with anexperimental investigation to augment the plate separator principle toincrease the separation and separation efficiency.

THEORETICALCONSIDERATION

Gravity separation is the most common treatment procedure. It is pri-marily based on the specific gravity difference between water andimmiscible oil globules and is used to move free oil to the surface of thewater, which is subsequently skimmed and removed. The API has spe-cified design criteria for simple gravity separators based on the removalof free oil globules larger than 150 mm in diameter. The conventionalStokes’ law describes the rise rate of the oil globules as

Vr ¼gD2ðrw � r0Þ

18mð1Þ

The performance of a gravity separator depends on the proper hydraulicdesign and the detention time period for a given rise velocity of oil glo-bules. Longer retention times generally increase separation efficiency.The liquid detention time must be sufficient to permit oil droplets risingat a given velocity to come to the fluid boundary where they can beremoved by skimming. Gravity separation often includes extended platesurfaces to decrease the effective rise height.

EXPERIMENTAL

The schematic diagram of the experimental setup is shown in Figure 1. Itconsisted of an oil-water mixing tank, a separator (unbaffled, baffled with

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different inclination, and variable area baffled), and sample collectionarrangement.

The mixing tank was a cylindrical steel tank 0.3 m in diameter and0.5 m in length. The tank was fitted with a double-bladed agitator to mixthe oil and water. The baffled separator was a vessel 1.25 min in lengthwith a square cross-section of 0.24 m� 0.24 m. It was made of Perspexand supported vertically on a slotted angle frame. The overflow line wasspecially constructed for the smooth removal of the top layer withoutdisturbing the lower water-rich layer. The tank bottom contained a drainline and was kept inverted to maintain a distinct interface layer betweenthe top oil-rich layer and effluent stream. Baffles of dimension 0.2 m by0.15 m were inserted in a rectangular structure made with slotted alu-minium angles. The dimension of this structure was slightly smaller thanthat of the baffled vessel so as to fit closely inside the vessel but also toprovide a path for the larger oil droplets to rise to the surface, aftercoalescing at the underside of the baffles. The baffles were inserted at

Figure 1. Schematic diagram of the experimental setup with 90� baffles. A: stirrer; B: baffles;

D: drainage line; O: overflow oil line; BV: baffled separator; MV: mixing vessel; S1�S7:

sample collecting points, V1�V2: valves.

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different spacing and inclinations to study their effect on the oilseparation efficiency. The number of baffles incorporated was also depen-dent on the baffle spacing. The baffle spacing used for the experiment was4 cm to 16 cm. Experiments were also conducted with different angle ofinclination to the vertical (30� to 90�). Sampling ports were insertedbelow each baffle to get the concentration of the oil at these points.

For the variable area baffles the experiment setup was identicalexcept that the baffles having square central holes of two differentdimensions (15.0 cm� 15.0 cm and 7.5 cm� 7.5 cm) were stacked alter-nately. These baffles were placed at an angle of 90� to the vertical. Thebaffle spacing was also identical to the previous experiments, i.e., 4 cmto 16 cm.

In the experiments, 2� 1072 m3 oil and water (1 : 19), i.e., 5% dieseloil, was taken in the mixing tank and was then agitated thoroughly for

Figure 2. Drop size characteristics.

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about 30 min. To get uniform mixture, the agitator speed was keptconstant at either 1000 rpm or 1500 rpm (for different drop diameter).The mixture was then fed into the water-filled baffled separator bykeeping the agitator running at low speed to maintain the suspension.The flow rate of the mixture was controlled by the adjusting valve in thedrain line in such a way that the mixture feed took approximately4 min. During the flow of the mixture into the baffled tank, the waterwas drained simultaneously from the bottom drain line to maintain aparticular level in the tank. The liquid was then allowed to settle in thetank for 15 min to separate out the diesel oil and water during itsdownward flow from one baffle to the other. The separated oil dropletscoalesced at the underside of the baffle to form larger droplets. Thedroplet then rose to the surface and formed a thick, dark, visible layerof oil over the water. The oil-rich top layer was then withdrawnthrough the overflow line by a reverse flow of water through the bottomdrain line, and 1000 mL of the top layer was collected and allowed tosettle in a measuring cylinder. When the interface between oil and waterbecame distinct, the oil volume collected was noted. Its density deter-mined the concentration of the separated oil layer. Sample ports werebuilt into the separator to determine the collection effectiveness of each

Figure 3. Concentration distribution of the oil over the length of the separator.

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baffle. Under steady state conditions samples were drawn off todetermine the residual oil concentration at each level. Experimentalruns with identical procedures were carried out with different bafflespacing (4 cm to 16 cm) and also with different angle of inclination tothe vertical (0� to 60�). For comparison of the oil separation betweenbaffled and unbaffled tanks, experiments were also conducted in anunbaffled tank, keeping the same experimental conditions.

Measurement of average drop size of oil was carried out by collectingsamples in a shallow petri dish and immediately measuring the dropletdiameters using an Axioplan microscope (Zeiss, FRG).

RESULTSANDDISCUSSION

Oil Droplet Size

Initially the oil droplet size was measured using an Axioplan microscope.The average drop size was determined by the following formula:

Dav ¼P

niDiPni

ð2Þ

Figure 4. Variation of the baffle spacing on the concentration gradient of oil over the length

of the separator.

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Figure 2 shows the drop size characteristics for 5% diesel oil-watermixture.

Concentration Distribution Across the Length of the Separator

Figure 3 shows the concentration distribution of the oil over the length ofthe separator. As the distance from the bottom increases the concentra-tion of oil also increases. It is clear from the graph that the unbaffledvessel also follows the same trend but deviates significantly from thebaffled vessel. Figure 4 shows the effect of baffle spacing on the con-centration gradient of oil across the length of the separator. It may beseen that the effect of baffle spacing is very negligible.

Effect of the Baffle Spacing on the Oil Separation

Figure 5 shows the effect of baffle spacing on the oil separation. It is clearfrom the figure that as the baffle spacing decreases, the percentagerecovery of oil increases, i.e., indicating higher oil separation. As bafflespacing decreases, the height of rise of the oil droplets to reach theunderside of the baffles decreases, which leads to the increase in oil

Figure 5. Variation of the baffle spacing on the oil separation.

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Figure 6. Variation of the baffle spacing on the oil separation for different systems.

Figure 7. Variation of angle of inclination on the oil separation.

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recovery. The separation is maximum at the baffle spacing of 4 cm, i.e.,one-sixth of the column width.

Figure 6 shows the effect of baffle spacing on the oil separation witha variable area baffle and compares the result with the 45� baffle. It wasalso observed that as baffle spacing decreases the oil recovery increases,and efficiency of the oil recovery is higher with 45� baffles.

Effect of Baffle Inclination on the Oil Separation

The effect of angle of inclination on the percentage recovery of oil isshown in Figure 7. It is clear from the figure that the percentage recoveryof oil increases as the baffle inclination to the vertical plane increasesfrom 30� to 45�, attains a maximum at 45�, and then decreases as theangle increases from 45� to 90�. This can be explained by the fact that at

Figure 8. Variation of angle of inclination on the oil separation for different oil droplet dia-

meters.

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45� inclination, entrained oil droplets are projected a velocity thatfollows the maximum tortuous path from the edge of the baffles. Thiscreates maximum possible separating force, which in turn leads toincrease in separation efficiency.

Effect of Droplet Size on the Separation

Figure 8 shows the effect of oil recovery on angle of inclination for twodifferent systems. It is clear from the figure that as the droplet sizeincreases the separation increases. Large droplets are easier to coalesceand form much bigger droplets than the small droplets. The risingvelocity of the oil droplets follows Stokes’ law in the laminar region.Since the rising velocity is directly proportional to the square of thediameter of the droplet, large drops are easier to separate.

CONCLUSION

Experiments on separation of 5% oil from an oil-water mixture havebeen carried out in unbaffled, baffled (with different inclination of baffle),and variable area baffled separators. The effect of baffle inclination,baffle spacing, and droplet size on the separation is reported. A baffledseparator with 45� inclination to vertical has been found to be the mostsuitable for the 5% oil-water separation.

NOMENCLATURE

D diameter, mm

g gravitational acceleration, m=s2

n number, dimensionless

V velocity, m=s

Greek lettersm viscosity, Pa.s

r density, kg=m3

Subscriptsi ith number

o oil

r rise

w water

av average

ACKNOWLEDGEMENT

S. K. Das is grateful to INSA for awarding the INSA Visiting ResearchFellowship in 1999�2000.

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