tust-2009-9

Research Article

Study of Air-Liquid Flow Patterns inHydrocyclone Enhanced by Air Bubbles

In order to improve the oil-water separation efficiency of a hydrocyclone, a newprocess utilizing air bubbles has been developed to enhance separation perfor-mance. Using the two-component phase Doppler particle analyzer (PDPA) tech-nique, the velocities of two phases, air and liquid, and air bubble diameter weremeasured in a hydrocyclone. The air-liquid mixing pump can produce 15 to60 lm-diameter air bubbles in water. There is an optimum air-liquid ratio foroil-water separation of a hydrocyclone enhanced by air bubbles. An air core oc-curs in the hydrocyclone when the air-liquid ratio is more than 1%. The veloci-ties of air bubbles have a similar flow pattern to the water phase. The axial andtangential velocity differences of the air bubbles at different air-liquid ratio aregreater near the wall and near the core of the hydrocyclone. The measured resultsshow that the size distribution of the air bubbles produced by the air-liquid mix-ing pump is beneficial to the process where air bubbles capture oil droplets in thehydrocyclone. These studies are helpful to understand the separation mechanismof a hydrocyclone enhanced by air bubbles.

Keywords: Air bubble enhancement, Hydrocyclone, Oil-water separation, Phase Dopplerparticle analyzer (PDPA)

Received: October 08, 2008; revised: October 17, 2008; accepted: October 20, 2008

DOI: 10.1002/ceat.200800518

1 Introduction

Hydrocyclones have been used for industrial purposes formore than 100 years. With considerable efforts in research anddevelopment, hydrocyclones are now widely used in variousindustries to separate two components of different densitieswith the aid of the strong centrifugal force created by the swir-ling flow.The flow inside a hydrocyclone is a complicated three-di-

mensional swirling flow. The study of the flow pattern in a hy-drocyclone is very important for the optimum design andevaluation of its separation performance. Since the separationmechanisms are associated with the velocity field, its exact de-termination is crucial for the understanding of the separationprocess [13]. Many studies have been conducted to under-stand the internal flow pattern of a hydrocyclone. The velocityprofiles of hydrocyclones were first measured by Kelsall [4],who used a stroboscope with a rotating microscope objective

lens to determine the velocities of aluminum flakes that seededthe flow in a transparent test section. Then Ohashi and Maeda[5] employed photographic techniques, which flashed at pre-cisely-controlled time intervals to determine the velocity of theseed particle. More recently, a number of authors used laserDoppler velocimetry (LDV) to determine the tangential andaxial velocities in hydrocyclones [69]. With the developmentof science and technology, mathematical models based oncomputational fluid dynamics (CFD) are highly desirable. Thefirst successful work in predicting the fluid flow in hydrocy-clones was achieved by Pericleous and Rhodes [10]; using thePHOENICS computer code for the solution of the partial dif-ferential equations, including the simple Prandtl mixing lengthmodel and the axisymmetry assumptions, the authors reportedthe velocity predictions in a 200-mm hydrocyclone. Later,Hsieh and Rajamani [11] numerically solved the turbulentmomentum equations to obtain the velocities and comparedthem with the laser Doppler velocimetry measurements in a75-mm hydrocyclone. In recent years, some works on the sim-ulation of hydrocyclones using the incompressible Navier-Stokes equations, supplemented by a suitable turbulence mod-el, have proven to be appropriate for modeling the flow in ahydrocyclone [1216].In order to improve the oil-water separation performance,

we enhanced the separation efficiency of conventionally con-

2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com

Zhi-shan Bai1

Hua-lin Wang1

Shan-Tung Tu1

1School of Mechanical andPower Engineering, East ChinaUniversity of Science andTechnology, Shanghai,P.R. China.

Correspondence: Prof. H. Wang ([email protected]), Key Lab ofSafety Science of Pressurized Systems, Ministry of Education, School ofMechanical andPowerEngineering, East ChinaUniversity of Science andTechnology, Shanghai, 200237, P.R. China.

Chem. Eng. Technol. 2009, 32, No. 1, 5563 55

structed hydrocyclones by using air bubbles. However, up totoday, no measured data is available for an air-liquid two-phase flow field in a hydrocyclone. The purpose of the presentwork is to obtain detailed knowledge of the air-liquid two-phase flow characteristics in a hydrocyclone using the phaseDoppler particle analyzer (PDPA) technique which is necessaryto understand the separation mechanism of the hydrocycloneenhanced by air bubbles.

2 Experimental Setup and Procedure

2.1 Hydrocyclone

A 35 mm-diameter hydrocyclone with two symmetrical, rec-tangular inlets (5 mm 10 mm) was used for the laser Dop-pler measurements. The dimensions of the hydrocyclones usedin the experimental work are shown in Tab. 1.

2.2 Air-Liquid Mixing

As shown in Fig. 1, air and liquid are mixed by the air-liquidmixing pump technology from Japanese NIKUNI Company.The maximum air-liquid ratio of the pump is about 1:9. Airenters the pump through the air inlet and is mixed and brokenup in the pump. The rejected air volume is controlled by avalve.

2.3 Flow Configuration

The flow diagram of oil-water separation using a hydrocycloneenhanced by air bubbles is illustrated in Fig. 2. Oil enters thepump by atmospheric pressure and is mixed with water in thepump. The pump generates small air bubbles and provideshigh probability for oil/bubble interaction. Water, oil, and airbubbles are fed into the hydrocyclone together. Oil dropletscollide with bubbles and were carried by bubbles in the hydro-

cyclone, the detailed process of which is shown in Fig. 3. Gasmoves faster than oil due to their differences in density andcan lead to interaction between oil and air [1720]. The inter-action between air bubbles and oil droplets can be summarizedas an entrapment of the air bubbles by a single or flocculatedstructure of oil droplets. So the oil can be removed more easilyand rapidly with the air bubbles than without in the hydrocy-clone. Larger diameter air bubbles were removed from thewater in the air-liquid separation pot because they can causedeterioration of the separation performance.In order to acquire air-liquid two-phase flow characteristics

in the hydrocyclone, water was the working fluid. Water(20 C) with small air bubbles was fed into the hydrocycloneunder controlled flow conditions by a set of valves. The fluidwas pumped from a 1.5-m3 water pot by means of an air-liq-uid mixing pump. Feed pressure and flow rates were moni-tored with accurate manometers and a flowmeter.Hollow glass microspheres were added to the water to gener-

ate tracer particles, which can provide sufficient water phaseflow seeding. Air bubbles can reflect light well and, therefore,no additional seeding was needed.

2.4 PDPA System and Measurement Procedure

A two-component phase Doppler particle analyzer (PDPA)was adapted for measuring. The base unit included a Spectra-Physics Stabilite 2017 ion laser operating in multiline modeand a fiber drive from Aerometrics. A Bragg cell inside the fi-ber drive split the laser beam in the 0 and 1 orders, whichwere then separated by two prisms into the different wave-lengths, thus providing two blue (488.0 nm) and two green(514.5 nm) laser beams. The laser beams were transmitted bymonomode polarization preserving glass fibers to the PDPAprobe. The original transmitting/receiving lens was substitutedby dedicated backscatter receiving optics. This setup allows si-


Table 1. Dimensions of the hydrocyclone used in the experimen-tal work.

Do/D Du/D Ls/D L/D Lu/D h

0.07 0.23 1.00 7.37 9.71 6

Water with air

Air inlet

Water inlet

Air bubbles

Figure 1. Air-liquid mixing.

1

2

34

5

6

7

7

7

8

8

8

9

Figure 2. Diagram of oil-water separation using hydrocyclonesenhanced by air bubbles: 1 Water pot; 2 Wastewater pot; 3 Oil pot; 4 Air flowmeter; 5 Air-liquid separation pot; 6 Air-liquid mixing pump; 7 Liquid flowmeter; 8 Pressure gauge;9 Hydrocyclone.

56 Z.-s. Bai et al. Chem. Eng. Technol. 2009, 32, No. 1, 5563

multaneous two-component velocity measurements in thebackscatter mode.Light collected by the transmitting/receiving lens was fo-

cused by an achromatic lens onto the end of a multimode fiberoptics. The collected light was separated into the individualwavelengths by a Dantec color separator and then transmittedto two Dantec photomultipliers. The PDPA signals were thenprocessed by two Dantec 57N20 BSA enhanced units, operat-ing in synchronous mode. Fig. 4 shows a block diagram of thePDPA system.Because the surfaces of the test section were cylindrical and

conical in shape, the pair of laser beams bend asymmetricallywhen they cross the inside and outside surfaces. This bendingtook place in all three dimensions because of the three-dimen-sional shape of the test model. The asymmetrical bending ofthe laser beams at the inside and outside surfaces of the testsection affected the probe volume position, the separation an-gle of the beams, and the direction of the measured velocity. Inorder to overcome the asymmetrical bending phenomena, a5 mm-diameter orifice vertical to the hydrocyclone axial wasopened in the position of the laser beams entrance and wasthen stopped up with optical glass. This is shown in Fig. 5.The optical ports can avoid the asymmetrical bending phe-nomena because of lower refraction ratio and plane surface.The laser route is illustrated in Fig. 6.

In order to study the effect of opticalports on the flow inside the hydrocy-clone, the computational fluid dy-namics method was used to simulatethe flow fields inside the hydrocyclonewith optical ports. In this work, thefluid flow is modelled as turbulent, de-scribed by the Reynolds stress model(RSM). The simulated results showthat the optical ports have little influ-ence on the internal flow of axial halfplanes without optical ports inside thehydrocyclone.During the measurements, the probe

was moved by using a highly accuratethree-component (xyz) traverse sys-tem controlled by a computer. Velocity

data were measured for the ten axial half planes without opti-cal ports from inlet to exit as defined in Fig. 7. For a given ax-ial coordinate, the first measurement point was placed nearthe wall of the cyclone and the last point was chosen in prox-imity of the core. The first axial step was equal to 20 mm fromthe top of the hydrocyclone. The other axial step was equal to30 mm and radial steps of 0.5 mm were applied. Finally, in allthe experimental work, a sample of typically 1000 Dopplerbursts was taken for each measurement. The experimentaldevice, including testing model and system, are shown inFig. 8.


Figure 3. Oil/bubble attachment [18].

3

2

1

Lase

Transmitting lens Receiving lens

Bragg cell Model

ProcessoComputer

Figure 4. Schematic of the PDPA system.

Orifice

Figure 5. Model of testing.

LaserLaser

Without optical port With optical port

Figure 6. The laser route.

Chem. Eng. Technol. 2009, 32, No. 1, 5563 Oil-water separation 57

3 Separation Performance of theHydrocyclone Enhanced by Air Bubbles

The Reynolds number of the inlet flow in the hydrocyclone isdescribed as follows:

Re qDvl

(1)

where

v 4QipD2

In the hydrocyclone, the air-liquid ratio could be defined asfollows:

Qq QgQl

(2)

The flow diagram of oil-water separation using the hydrocy-clone enhanced by air bubbles is illustrated in Fig. 2. The in-fluence of air-liquid ratio, Qq

1), on separation efficiency, E, isillustrated in Fig. 9. When the air-liquid ratio is less than 1%,an increase in the air-liquid ratio will improve the efficiency ofoil removal. The efficiency reaches a maximum when the air-liquid ratio is close to 1%, beyond which further increases in

air-liquid ratio will cause performancedeterioration. This is caused by the fol-lowing: A certain air bubble number in liq-uid is advisable to reduce the viscosi-ty of the liquid and to raise the colli-sion probability of oil with water. Itwill make the removal of oil moreeasily.

For a high air-liquid ratio, the airbubbles can also reduce the viscosityof the liquid and raise the collisionprobability of oil with water, but ex-cessive air bubbles disturb the flowfield in the hydrocyclone and de-creases the separation efficiency.The air-liquid ratio of 1% can pro-

vide a better separation of oil fromwater. The experimental results showthat air bubbles that occur in water canimprove the deoiling efficiency of hy-drocyclones.

4 Results and Discussion

4.1 Size Distribution of AirBubbles

In order to acquire the high oil-waterseparation performance using the hydrocyclone enhanced byair bubbles, the size distribution of air bubbles in water is animportant factor. According to the appropriate literature [1719], the oil droplets in oily wastewater are normally stabilizedas an oil-in-water emulsion, having a median droplet diameterusually in the range of 350 lm. A range of air bubbles is ben-eficial because the smaller bubbles can capture the smaller oildroplets and the larger ones, the larger droplets [2628]. Thedistribution of air bubble diameter inside the hydrocyclone isplotted in Fig. 10 under the condition of the inlet flow rate of


20 30 30 30 30 30 30 30 30 30

Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Z10

Figure 7. The dispose of testing plane.

Optical port

Hydrocyclone

(a) Model of testing (b) Testing system

Figure 8. Photograph of the experimental system.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.572

76

80

84

88

92

E /%

Qq

Figure 9. Separation efficiency (E) vs. air-liquid ratio (Qq) (Oilconcentration 100 mg/L; inlet Reynolds number 15000).

1) List of symbols at the end of the paper.


1.50 m3/h and Qq = 1.0%. It can be seen that the air-liquidmixing pump can produce 1560 lm-diameter air bubbles inwater. The size distribution of air bubbles produced by the air-liquid mixing pump is beneficial to the process where air bub-bles capture oil droplets in the hydrocyclone.Fig. 11 shows the distribution profiles of air bubble size in

the hydrocyclone when Qq = 1.0%; the inlet flow rate at

1.50 m3/h. It indicates that the diameter of air bubbles increasefrom the wall towards the center. The size distributions remainsimilar at different axial heights in the body of the hydrocy-clone. The diameter change diminishes at radial distance withthe increase of axial height. At an axial height of 260 mm,there is no diameter change at any of the radial distances. Thisresult is in agreement with previous theoretical studies on sizedistribution [4].

4.2 Velocity of Water

For the inlet flow rate of 1.50 m3/h, the axial velocity profilesat different radial distances and at different axial heights in thehydrocyclone are integrated and presented in Fig. 12a). Theaxial velocity is downward, i.e., negative, close to the hydrocy-clone wall and upward towards the center. It can be observedfrom the figure that a peak positive axial velocity is immedi-ately below the bottom of the vortex finder and a minimumvalue of positive axial velocity is at a vertical distance of170 mm from the top of the hydrocyclone. The figure also in-dicates that at an axial height (Z) of 200 mm, there is no posi-tive vertical velocity at all the radial distances, indicating nofurther classification of water in this region. With increasingaxial distance, the positive values of axial velocity decreases tillit reaches zero at some axial distance. At an axial height of


Figure 10. Size distribution of air bubbles.

20m

360

330

300

270

240

210

180

150

120

90

60

30

0

-300 5 10 15 20

r/mm

Z/m

m

Figure 11. Granularity distribution of air bubbles.

r

(a) Axial velocity (b) Tangential velocity360

330

300

270

240

210

180

150

120

90

60

30

00 5 10 15 20

r/mm

Z/m

m

360

330

300

270

240

210

180

150

120

90

60

30

0

-300 5 10 15 20

/mm

Z /m

m

5m/s

5m/s

Figure 12. Velocity distribution of water.


200 mm, there is no positive axial velocity at any radial dis-tances, indicating no further classification in this region. Thedistributions of axial velocity indicate that the main separationparts are the cylinder and the upside of the cone. The geome-try of the cylinder and cone angle is key to the design of a hy-drocyclone.The measured results of tangential velocity at different verti-

cal heights are plotted in Fig. 12b) under the condition of inletflow rate at 1.50 m3/h. It can be observed from the figure thatthe tangential velocity increased from the hydrocyclone walltowards the center, reached a maximum value, and then rapid-ly decreased. The profiles of tangential velocity remain similarat different axial heights in the body of the hydrocyclone. Max-imum values of tangential velocities are observed in the cylin-drical portion. It can also be noted that the maximum valuesof tangential velocity decrease with decrease in axial height.

4.3 The Effects of Air-Liquid Ratio on Velocity ofWater

For an inlet flow rate of 1.50 m3/h, the axial velocities at differ-ent Qq are shown in Fig. 13. It can be seen from the figure thatthe axial velocity increases in the core region with the increaseof Qq. This is because the air content is higher in the core thanother regions due to centrifugal force. It is beneficial to removeoil droplet bypass overflow. The change rule of axial velocity isnot uniform with the increase in Qq in other regions.

The tangential velocities at different Qq are shown inFig. 14. It can be seen from the figure that Qq affects tangentialvelocity only in the core of the hydrocyclone; there is littlechange in other regions. The axial velocity increases in the coreregion with the increase in Qq.It can be seen from Fig. 13 and Fig. 14 that the air core oc-

curs in the hydrocyclone when the air-liquid ratio is more than1%. There is an optimum air-liquid ratio for oil-water separa-tion of a hydrocyclone enhanced by air bubbles. At the sameinlet flow rates, an increase in air-liquid ratio will improve theseparation efficiency under the condition of the air-liquid ratiobeing less than the optimum split ratio, beyond which furtherincrease in the air-liquid ratio will decrease the separation per-formance. This is because at higher air-liquid ratio, an air coreoccurs in the hydrocyclone, large numbers of air bubbles willarouse turbulence fluctuation and then make it possible for fi-ner oil droplets to move into the underflow, thus reducing theseparation efficiency.

4.4 Velocity of Air Bubbles

The axial velocity distributions of air bubbles at different Qqare shown in Fig. 15. It can be seen that the axial velocity dis-tributions of air bubbles have a similar trend to the waterphase. The tangential velocity distributions of air bubbles atdifferent Qq are shown in Fig. 16. It can also be noted that themaximum values of tangential velocity decrease with decrease


Z1

0 2 4 6 8 10 12 14 16-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

Qq=0.5%Qq=1.0%Qq=2.0%

u Z/m

.s-1

r/mm

Z3

Z5 Z7

0 2 4 6 8 10-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Qq=0.5%Qq=1.0%Qq=2.0%

u Z/m

.s-1

r/mm0 2 4 6 8 10 12 14

-1.5

-1.0

-0.5

0.0

0.5

1.0

Qq=0.5%Qq=1.0%Qq=2.0%

u Z/m

.s-1

r/mm

0 2 4 6 8 10 12 14 16 18 20-1.2

.8

.4

0.0

0.4

8

21.

Qq=0.5%0.Qq=1.0%

-0

-0

Qq=2.0%

u Z/m

.s-1

r/mm

Figure 13. Axial velocity distribution of water at different Qq.



0 2 4 6 8 10 12 14 160

-1

-2

-3

-4

-5

-6

-7

-8

Qq=0.5%Qq=1.0%Qq=2.0%

u /m

.s-1

r/mm

0 2 4 6 8 10 12 14 16 18 20-1

-2

-3

-4

-5

-6

-7

-8

Qq=0.5%Qq=1.0%Qq=2.0%

u /m

.s-1

r/mmZ3Z1

0 2 4 6 8 10-3-4-5-6-7-8-9

-10-11-12

Qq=0.5%Qq=1.0%Qq=2.0%

u /m

.s-1

r/mm0 2 4 6 8 10 12

0

-1

-2

-3

-4

-5

-6

-7

-8

Qq=0.5%Qq=1.0%Qq=2.0%

u /m

.s-1

r/mm

Z5 Z7

Figure 14. Tangential velocity distribution of water at different Qq.

0 2 4 6 8 10 12 14 16 18 20-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

Qq=0.5%Qq=1.0%Qq=2.0%

u Z/m

.s-1

r/mm

Z1

0 2 4 6 8 10 12 14 16-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

Qq=0.5%Qq=1.0%Qq=2.0%

u Z/m

.s-1

r/mm

Z3

0 2 4 6 8 10-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Qq=0.5%Qq=1.0%Qq=2.0%

/m.s-

1u Z

r/mm

Z5

0 2 4 6 8 10-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Qq=0.5%Qq=1.0%Qq=2.0%

u Z/m

.s-1

r/mm

Z7

Figure 15. Axial velocity distribution of air bubbles.


in axial height. The magnitude of the tangential velocity valueincreases at the same radial distance with the increase in Qq.The tangential velocity distributions of air bubbles have a simi-lar trend to the water phase.The axial and tangential velocity differences at different Qq

are greater near the wall and near the core of the hydrocyclone.The velocity differences are greater near the core because thevelocity and velocity gradients at the core region are greaterand then arouse the turbulence fluctuation. The collision be-tween wall and fluid make the velocity fluctuations strengthennear the wall.

5 Conclusions

By using a two-component phase Doppler particle analyzer(PDPA) technique, the velocities of two phases, air and liquid,were measured in a hydrocyclone enhanced by air bubbles.The results are summarized as follows: In order to overcome the influence of shape on laser beams,optical ports were set up in the hydrocyclone and the asym-metrical bending phenomena were resolved.

Under the condition of the inlet flow rate of 1.50 m3/h andQq = 1.0%, the air-liquid mixing pump can produce 15 to60 lm-diameter air bubbles in water. There is an optimumair-liquid ratio for oil-water separation of a hydrocycloneenhanced by air bubbles. At higher air-liquid ratios, large

numbers of air bubbles will reduce the separation efficiency.The size distribution of air bubbles produced by air-liquidmixing pump is beneficial to the process that air bubblescapture oil droplets in the hydrocyclone.

The velocities of air bubbles have a similar flow pattern tothe water phase. The axial and tangential velocity differencesof air bubbles at different air-liquid ratios are greater nearthe wall and near the core of the hydrocyclone.This work is helpful to understand the separation mecha-

nism of a hydrocyclone enhanced by air bubbles. However, inorder to achieve an optimal design, further studies are consid-ered in two directions: (1) The oil-water-air three-phase velo-cities in hydrocyclones should be examined; (2) the interac-tions of air bubbles and oil droplets should be studied.

Symbols used

D [mm] hydrocyclone diameterDo [mm] overflow orifice diameterDu [mm] underflow orifice diameterE [%] separation efficiencyL [mm] length of coneLs [mm] length of swirl chamberLu [mm] length of tail pipeQg [m

3/h] air and liquid flow rateQi [mm] inlet flow rate


0 2 4 6 8 10 12 14 16-2

-3

-4

-5

-6

-7

-8

Qq=0.5%Qq=1.0%Qq=2.0%

u /m

.s-1

r/mmZ3

0 2 4 6 8 10 12 14 16 18 20-1

-2

-3

-4

-5

-6

-7

-8

-9

Qq=0.5%Qq=1.0%Qq=2.0%

u /m

.s-1

r/mm

Z1

0 2 4 6 8 10 12 140

-1-2-3-4-5-6-7-8-9

Qq=0.5%Qq=1.0%Qq=2.0%

u /m

.s-1

r/mm

Z5

0 2 4 6 8 10-3

-4

-5

-6

-7

-8

-9

-10

-11

Qq=0.5%Qq=1.0%Qq=2.0%

u /m

.s-1

r/mm

Z7

Figure 16. Tangential velocity distribution of air bubbles.


Ql [m3/h] liquid flow rate

Qq [] air-liquid ratioRe [] Reynolds numberuz [m/s] axial velocityuh [m/s] tangential velocitym [m/s] hydrocyclone characteristic velocityZ [mm] axial height from top wall

Greek symbols

q [kg/m3] density of liquidh [] cone angle

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