International Journal of Mining Science and Technology 23 (2013) 665–668

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International Journal of Mining Science and Technology

journal homepage: www.elsevier .com/locate / i jmst

Experimental investigation and modeling of flotation column fortreatment of oily wastewater

2095-2686/$ - see front matter � 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology.http://dx.doi.org/10.1016/j.ijmst.2013.08.008

⇑ Corresponding author. Tel.: +86 516 83885878.E-mail address: cumtlxb@126.com (X. Li).

Ran Jincai, Liu Jiongtian, Zhang Chunjuan, Wang Dengyue, Li Xiaobing ⇑National Engineering Research Center of Coal Preparation and Purification, China University of Mining & Technology, Xuzhou 221116, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 December 2012Received in revised form 18 January 2013Accepted 28 February 2013Available online 5 September 2013

Keywords:Oily wastewaterSeparationFloatation columnModeling

A unique cyclonic static microbubble flotation column was developed for oily wastewater separation. Theseparation efficiency was found to be highly dependent on gas holdup and bubble size distribution. Bychanging the circulation pressure, gas flow rate, frother concentration, the effect of operation parameterson gas holdup and oil removal efficiency were attained. A mathematical modeling between the kineticconstant and the gas holdup was established for oily water separation process. The results show thathigher gas holdup and smaller microbubble sizes are beneficial to improve oil removal efficiency.

� 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction

A large amount of oily wastewater is produced during crude oilexploration and production, which usually contains a complexmixture of many hydrocarbons and crude oil, suspended solids,salts, polymers and surfactants. A microbubble-based separationof oily wastewater is widely used because of its high efficiencyand low cost [1]. Using a flotation cell or column, microbubblesgenerated by bubble generator system and oil droplets formedthe air/oil droplet aggregates after collecting and adhering, thenrising to the top of flotation units, as a result of oil separated fromwastewater.

The flotation separation of oily wastewater usually requiresmicrobubbles, quiescent hydrodynamic conditions in the columnor cell collection zone. A microbubble swarm is produced bybubble generator, which has a profound effect on flotationseparation efficiency, and the flotation rate and efficiency are alsoaffected by the bubble size distribution [2]. A bubble size measure-ment technique has been investigated by using the photographictechnology, taking Matlab language as the programming environ-ment [3].

The gas holdup (eg) refers to the composition percentage of gasvolume in the flotation column, which is mainly the volume ratioof replacement liquid after bubbles enter the flotation column [4].

eg ¼Vg

Vg þ Vlð1Þ

where eg is the gas holdup; and Vg, Vl the volumes occupied by gasand liquid in the flotation column, respectively.

The gas holdup in the collection zone of a flotation column is animportant variable which can affect oil droplet residence time andcollection. Gas holdup depends on the gas flow rate, bubblediameter, circulation pressure, frother concentration and othervariables [5–7].

In this paper, a modeling of this novel cyclonic static microbub-ble flotation column with an internal conventional hydrocyclonewas developed for flotation treatment of oily wastewater. Theeffects of operational factors on gas holdup and separationefficiency of oily wastewater were investigated, and a mathemati-cal model of the relationship between oil removal and gas holdupwas established. This model will be used to improve the flotationefficiency.

2. Materials and process description

2.1. Materials

The oily wastewater samples used in this study were obtainedfrom Shengli Oilfield. The wastewater has the following properties:pH 7.44; density of 1.003 � 103 kg/m3; viscosity of 1.2238 mPa�s;oil concentration of 1616.53 mg/L; hydrolyzed polyacrylamideconcentration of 152.20 mg/L; and suspended solid concentrationof 130.00 mg/L. The wastewater temperature was kept at 39 �C.The oil droplet size distribution was analyzed with a laser particleanalyzer (IO64L, France Cilas Company). The oil droplets of 100 lmor smaller represented 85.59% of total oil droplets.

0.5 1.0 1.5 2.0 2.5 3.0 3.53.03.54.04.55.05.56.06.57.07.58.08.5

Gas holdup

Removal efficiency

Gas flow rate (m3/h)

Gas

hol

dup

(%)

60

65

70

75

80

85

90

Rem

oval

eff

icie

ncy

(%)

0

Fig. 2. Effects of gas flow rate on gas holdup and oil removal efficiency.

666 J. Ran et al. / International Journal of Mining Science and Technology 23 (2013) 665–668

2.2. Process description

A schematic diagram of the experimental apparatus used in thisstudy is shown in Fig. 1.

A pilot-scale cyclonic static microbubble flotation column wasmade of a 150 mm diameter, and 3500 mm height plexiglas cylin-drical column. Feed wastewater was pumped through water dis-tributor of the flotation column, a valve to control the feed rate.Clean water from the bottom of flotation column discharge ratewas controlled with another valve. Effluent from the bottom ofinternal cyclone of flotation column was pumped continuously tothe top of the internal cyclone, and oily wastewater with low sep-arability was forced to cyclonic separate which was kept under thecontrol of a circulation pump. Gas was introduced into the top ofthe internal cyclone in flotation column through the microbubblegenerator. The flow rate of gas and liquid were measured withgas flowmeter and liquid flowmeter. Two silicon pressure trans-mitters were installed at positions with distances of 830 mm and1830 mm from the top of column, respectively. The ICP/DASI-7017C analog input module and I-7520 communication modulewere selected and connected with the computer so that the datafrom these two pressure transmitters could be recorded by onlineand in real time.

3. Results and discussion

3.1. Effects of factors on separation of oily wastewater

3.1.1. Effects of gas flow rateIn the flotation process, the basic mechanism of oil droplet

collection consists of collision and attachment of the oil dropletson the interfacial microbubble surface. Subsequently, air isrequired to transport the floatable oil droplets to the overflow. Ina flotation column, the higher gas flow rate is, the higher gas hold-up and oil removal efficiency are. Experimental conditions are asfollows: the circulation pressure of 0.16 MPa, without frother.The effects of gas flow rate on gas holdup and oil removal efficiencyare presented in Fig. 2.

Fig. 2 presents that the oil removal efficiency is increasing withan increase of the gas flow rate. However, a slowly decrease in oilremoval efficiency was observed with a gas flow rate of 2.00 m3/h.In this process, microbubbles are required, and the medium of oil

Clean water

Oily water

Air

Mixing tank

Feed pump

Valve

Communication module

Input module

Computer

Clean water tank

Flotation column

Circulation pump

Gas

flo

wm

eter

Liq

uid

flow

met

er

Pressure gauge

Microbubble generator

Cyclone

Water distributor

Silicon pressure transmitter

Fig. 1. Cyclonic static microbubble flotation column process flowsheet.

droplet transport is the bubble surface. Consequently, the increaseof the gas flow rate promotes flotation through the larger numberof microbubbles. The microbubbles increase with the increasingrate of gas flow, which obtains a high probability of oil droplet/bubble collisions and adhesions, resulting in high oil removal effi-ciency. However, a high degree of turbulence occurs when the gasflow rate increases above optimum gas flow rate, and quiescenthydrodynamic condition is destroyed in the column, resulting indecrease of oil removal efficiency. Fig. 2 also shows that the gasflow rate increases from 0.50 to 3.0 m3/h and the gas holdupincreases from about 3.55% to 7.64%.

3.1.2. Effect of frother concentrationBubbles produced in water are unstable. One of the prerequi-

sites for a successful flotation separation of oily wastewater isthe stability of the oil droplet/bubble aggregate. A frother is a mol-ecule which is able to modify the properties of an interface ofwater/air or oil/water by reducing the surface or interfacial tensionof the liquid/air or oil/water in order that a stable bubble is pro-duced in the system. Moreover, the frother influences the kineticconstant of oil droplet/bubble adhesion, and stabilizes the oil drop-let/bubble aggregate. By adjusting the frother concentration, high-er gas holdup and smaller bubble can be obtained. Experimentalconditions are as follows: circulation pressure of 0.16 MPa andgas flow rate of 1.50 m3/h. The effects of frother concentration ongas holdup and oil removal efficiency are presented in Fig. 3.

The gas holdup and oil removal efficiency depend on the con-centration of the frother. Fig. 3 displays that with an increase inthe frother concentration, the gas and oil removal efficiencyincrease. Fig. 3 also shows that the concentration of the frotherincreases from 2.00 to 20.00 mg/L, the gas holdup increases from11.47% to 12.35%, and the oil removal efficiency increases from86.56% to 93.58%. In this process, the increase of the frother con-centration promotes flotation through an enlarged bubble surface,a decreased bubble size and an increased bubble quantity.

2 4 6 8 10 12 14 16 18 20

11.4

11.6

11.8

12.0

12.2

12.4

Frother concentration (mg/L)

80

82

84

86

88

90

92

94

0

Gas

hol

dup

(%)

Rem

oval

eff

icie

ncy

(%)

Gas holdupRemoval efficiency

Fig. 3. Effects of frother concentration on gas holdup and oil removal efficiency.

0.10 0.14 0.18 0.22 0.26 0.304.0

4.24.4

4.6

4.8

5.05.2

5.4

5.6

5.86.0

p (MPa)

60

65

70

75

80

85

90

Gas holdupRemoval efficiency

Gas

hol

dup

(%)

Rem

oval

eff

icie

ncy

(%)

Fig. 4. Effect of circulation pressure on gas holdup and oil removal efficiency.

J. Ran et al. / International Journal of Mining Science and Technology 23 (2013) 665–668 667

3.1.3. Effect of circulation pressureThe circulation pressure from the circulation pump is a key fac-

tor that affects the oil removal performance, which provided en-ergy to the flotation column for the oily wastewater separation.On the one hand, the circulation pressure can reflect the returnedquantity of oily wastewater which is difficult to be separated. Onthe other hand, the circulation pressure can also affect the size ofmicrobubbles from generator. Experimental conditions are asfollows: gas flow rate of 1.50 m3/h, without frother. The effects ofcirculation pressure on gas holdup and oil removal efficiency arepresented in Fig. 4.

Fig. 4 presents that the oil removal efficiencies gradually increasewith the increasing circulation pressure, and the removal efficiencyhas an optimum of circulation pressure at about 0.22 MPa, achiev-ing 84.69% removal efficiency. Thereafter, circulation pressure ishigher than 0.22 MPa, and the percentage decreases dramaticallydown to 80.38% oil removal. The concentration at which the oilremoval efficiency is found to be maximum is called the optimalcirculation pressure. The decrease in the oil separation efficiencyin the flotation column appears to be due to, at least, the followingphenomena: (a) a low probability of oil droplet/bubble collisionsand adhesions; (b) a low gas holdup; and (c) a high degree turbu-lence destroyed the quiescent environment.

Fig. 4 also shows that the circulation pressure increases from0.12 to 0.28 MPa, and the gas holdup decreases from about 5.80%to 4.52%. This is because smaller bubbles rise quickly and the res-idence time reduces in column. Thus, the optimum circulationpressure for this system was found to be 0.20 MPa.

3.2. Process kinetics

There is a wide divergence of views on the kinetic order offlotation separation process for the oily wastewater. It wasgenerally assumed that the overall flotation was described as a firstorder process with respect to the concentration of the floatable

10 20 30 40 50 60 70 80 90 100

10-1

100

Flotation time (min)

C/C

0

10 20 30 40 50

10-2

PL=0.20 MPa, Qg=1.50 m3/h, CF=10.00 mg/L

PL=0.15 MPa, Qg=1.50 m3/h, CF=0 mg/L

PL=0.25 MPa, Qg=1.50 m3/h, CF=20.00 mg/L

Flotation ti

10-1

100

C/C

0

10- 2

PL=0.1CF=10

PL=0.1CF=0

PL=0.1CF=20

(a) (b

Fig. 5. Oil concentration varied w

mineral/oil, so many researchers agreed that it was similar tochemical kinetics, which might be described as a first order rateequation [8]. Takahashi reported that the flotation experimentsshowed a deviation from the first order process and expressed adistribution of the rate constant k as [9].

CC1¼Z 1

0nðkÞ expð�ktÞdk ð2Þ

where n(k) is the distribution function of the rate constant; k the oildroplet removal rate constant; and C and C1 the t time and asymp-tote of oil concentrations, respectively.

In typical first order mineral/oil flotation, the collection rate(�dC/dt) is considered to be proportional to mineral/oil concentra-tion. The interaction among oil droplets is easily considered thatthis interaction increases with the increase of gas holdup and oilconcentration. Therefore, a modified kinetic modeling wassuggested for the oil flotation separation processes by Lai et al.[10].

� dCdt¼ k

C � C1t

ð3Þ

It was important to determine the process parameters and con-ditions or characteristics of the floatable particles that affect the oilremoval kinetics so as to improve the performance of flotation pro-cess [11]. In this study, the kinetic study was carried out underoperation conditions: circulation pressure (PL) of 0.15, 0.25,0.30 MPa, gas flow rate (Qg) of 1.50, 2.00, 2.50 m3/h, frother con-centration (CF) of 0, 10, 20 mg/L. For each condition, experimentwas carried out and each water sample was analyzed two timesso that average values for oil removal efficiency, gas holdup andbubble size were reported. Experimental results of oil removal effi-ciency are plotted in Fig. 5a–c. It is seen from the oil concentrationchange curve that the oil removal in the cyclonic static microbub-ble flotation column is efficient. The oil removal efficiency can beas high as 97.03% within 25 min of flotation.

3.3. Correlation of removal with hydrodynamic parameters

As defined in Eq. (3) the oil droplet removal rate constant k isobtained and the results are listed in Table 1. According to abovetests, the experimental results of gas holdup and bubble size arealso summarized in Table 1.

Circulation pressure of flotation column, gas holdup and aver-age diameter of bubbles were major factors. Thus, based on therelationship proposed by Gu Xuqing, the quantitative relationshipbetween constant k and other parameters has been attempted toelicit [12]

k ¼aeb

gPcL

Ddb

ð4Þ

60 70 80 90 100 10 20 30 40 50 60 70 80 90 100

me (min) Flotation time (min)

10-1

100

C/C

0

10-2

5 MPa, Qg=2.00 m3 /h, .00 mg/L

5 MPa, Qg=1.50 m3 /h, mg/L

5 MPa, Qg=2.50 m3 /h, .00 mg/L

PL=0.15 MPa, Qg=2.50 m3/h, CF=20.00 mg/L

PL=0.20 MPa, Qg=2.00 m3/h, CF=20.00 mg/L

PL=0.25 MPa, Qg=1.50 m3/h, CF=20.00 mg/L

) (c)

ith the operation parameters.

Table 1Summary of kinetic constants.

PL (MPa) Qg (m3/h) CF (mg/L) eg (%) Db (mm) k

0.15 1.50 0.00 5.72 3.40 0.620.15 2.00 10.00 8.76 1.32 1.080.15 2.50 20.00 12.35 0.73 1.470.20 1.50 10.00 8.93 1.06 1.200.20 2.00 20.00 11.82 0.52 2.070.20 2.50 0.00 6.41 2.46 0.720.25 1.50 20.00 11.72 0.69 1.740.25 2.00 0.00 6.53 2.42 0.740.25 2.50 10.00 9.01 1.27 1.18

Table 2Regressed constants.

a b c d

0.7195 0.2968 0.0335 0.5555

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Calculated values (k)

Exp

erim

enta

l val

ues

(k)

5%

+5%

Fig. 6. Comparsion of kinetic constants between calculated values and experimentsvalues.

668 J. Ran et al. / International Journal of Mining Science and Technology 23 (2013) 665–668

where k is the flotation kinetic constant; a, b, c, d the empiricalconstant; and eg, Db, PL the gas holdup, mean diameter of bubbleand circulation pressure, respectively.

According to Table 1, the empirical constants of a, b, c, d weredetermined by the least square method and the results are listedin Table 2.

The relationship between kinetic constant k and the circulationpressure, gas holdup and mean diameter of bubble for the oilywastewater flotation separation process can be demonstrated asfollows:

k ¼0:7195e0:2968

g P0:0335L

D0:5555b

ð5Þ

In Eq. (5) it shows that the separation performance is strongly af-fected by the hydrodynamic behavior of the flotation process. It isalso seen that an increase of gas holdup and circulation pressure,and a decrease of mean diameter of bubble will increase the kineticconstant k and improve separation efficiency of oily wastewater.Among the three parameters, the bubble diameter is the most

influential factor on the flotation efficiency (d > b, c). It is provedthat the generation of microbubble is important to improve the re-moval efficiency of fine droplets, and the effect of gas holdup is lar-ger than the circulation pressure. It also indicates that the gasholdup is a very influential parameter for oil removal. A comparisonof calculated and experimental k is shown in Fig. 6. The standard er-rors are within the ±5% zone.

4. Conclusions

(1) A novel cyclonic static microbubble flotation column with aninternal cyclone was developed, and effects of operationalparameters on gas holdup and oil removal were investi-gated. Under the optimal conditions: circulation pressureof 0.20 MPa, gas flow rate of 2.00 m3/h, and frother concen-tration of 20.00 mg/L, the gas holdup 11.82%, the diameter ofbubble decreasing to 0.52 mm, and improved removalefficiency to 97.03% within 25 min of flotation.

(2) The relationship between kinetic constant k and the circula-tion pressure, gas holdup and mean diameter of bubble wasestablished. Increasing the gas holdup or decreasing thediameter of the bubbles can result in an increase of thekinetic constant k and oil removal efficiency.

Acknowledgments

The authors are grateful to the National Natural ScienceFoundation of China (No. 51104158) and the Science andTechnology Fund of China University of Mining & Technology(No. 2011QNB08) for the financial support.

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