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Cyclonic separation process intensification oil removal based on microbubble flotation
International Journal of Mining Science and Technology 23 (2013) 415–422
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Cyclonic separation process intensification oil removal based on microbubble flotation Liu Jiongtian ⇑, Xu Hongxiang, Li Xiaobing Chinese 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
Article history: Received 2 October 2012 Received in revised form 25 October 2012 Accepted 10 November 2012 Available online 9 July 2013 Keywords: Cyclonic-static microbubble flotation column Microbubble flotation Cyclonic separation Oil–water separation
a b s t r a c t The cyclonic-static microbubble flotation column has dual effects including the cyclonic separation and floatation separation with the characteristics of the small lower limit of the effective separation size, short separation time, large handling capacity, and low operation cost. It shows significant advantages in the oily wastewater treatment field, especially the polymer flooding oily wastewater treatment aspect. In this paper, the cyclonic separation function mechanism of the cyclonic-static microbubble flotation column was studied, the impact of the parameters including the feeding rate, aeration rate, circulating pressure, and underflow split ratio on the cyclonic separation efficiency was investigated, and the cyclonic separation efficiency model was established as well. In addition, by applying the Doppler Laser Velocimeter (LDV) and Fluent simulation software, the test and simulation to the single-phase flow velocity field of the cyclonic separation section of the cyclonic-static microbubble flotation column were carried out, and the velocity distribution rule of the cyclonic separation section was analyzed under the singlephase flow conditions. Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Currently, many domestic oilfields have stepped into the medium and high water cut stage of the mid-to-late petroleum exploitation period; thus, the oily wastewater treatment has become one of the key challenges for the oilfield mine production and environment protection. Initially, the floatation is used to selectively separate the targeted mineral from the non-targeted mineral by foam. With advantages of high separation efficiency, large treating capacity and low cost, the flotation technology has become a kind of water treatment technology widely researched both at home and abroad, and has been gaining more and more popularity [1–8]. With wider application of the flotation technology in the oily wastewater treatment, many sets of flotation equipment have emerged correspondingly. The common flotation equipment covers the impeller suction floatation machine, air jet flotation machine, eccentric flotation machine, pneumatic flotation column, pneumatic cyclone, etc. [9–18]. Though such kinds of flotation equipment can show superior functions in pre-treatment of the water flooding liquid and wastewater treatment of the oilfield, bubbles arising from entrainment and back mixing as well as slow oil drop coalescence due to poor contact environment and smaller application range of the oil concentration for raw water are main parameters influencing the separation efficiency. In addition, with increased liquid production by
polymer flooding and ternary complex, the application of the common flotation equipment is also seriously restricted. Under this circumstance, researching and developing the efficient flotation equipment is important to the oilfield development. A kind of cyclonic-static microbubble flotation column with unique structures has been introduced to the oil–water separation field [19]. The cyclonic-static microbubble flotation column has also been applied to make the industrial test in Shengli Gudao Oilfield. The oil content of the crude oil, gas and water separation tank in the joint Station of Gudao Oilfield is 2000–2500 mg/L, which has been reduced to below 10 mg/L after being subject to the separation treatment by the flotation column. The cyclonic-static microbubble flotation column is a complicated separation process integrated with the common cyclonic separation and common foam flotation separation. The built-in cyclone of the cyclonic-static microbubble flotation column is a kind of unique gas-injected liquid–liquid hydraulic cyclone; on this account, the research is based on the forced oil removal process by cyclonic separation of the microbubble flotation; the oil–water separation efficiency of the cyclonic separation section of the cyclonic-static microbubble flotation column is significant to further improve the oil–water separation efficiency of the cyclonic-static microbubble flotation column. 2. Discussion of the cyclonic separation process
⇑ Corresponding author. Tel.: +86 516 83885878. E-mail address: [email protected] (J. Liu).
The cyclonic-static microbubble flotation column (Fig. 1) is integrated with the floatation column on the basis of the common
2095-2686/$ - see front matter Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.05.010
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Airflotation
oily wastewater quality of the No. 6 joint Station of Shengli Gudao Oilfield is poor; the non-settled is yellowish-brown, with logs of dispersed oil and suspended matter. Besides, due to high oil content and high content of suspended matters and polymers, the emulsion in the oily wastewater of the No. 6 joint Station of Shengli Gudao Oilfield was stable, which is too difficult to treatment.
Cyclonic
3.2. Experimental set-up
Fig. 1. Cyclonic-static microbubble flotation column.
static hydraulic cyclone, and the oil–gas complex which is generated by carrier effect of the self-suction bubble generator of the flotation column will transport the emulsified and fine-graded oil droplets in the oily wastewater to the column separation area from the cyclonic separation area; then by applying the static separation effect of the ‘‘long and narrow’’ environment with ‘‘quiet’’ fluid dynamics in the column separation area, the oil–water separation will be carried out [20]. Consequently, after being pressurized by the circulating pump, the oily wastewater with poor separability will enter into the built-in cyclone of the flotation column from the middle and lower parts of the flotation column, and perform further cyclonic separation under the effect of the centrifugal force field. As the cyclonic-static microbubble flotation column is integrated with the cyclonic separation technology and flotation separation technology, it has dual effects including the cyclonic separation and airfloatation separation. In addition, the synergistic effect of multiple separation modes reinforces the separation effect and expands the size range of the oil content in the oily wastewater for flotation separation. In addition to a higher treating efficiency, when being compared to the common hydraulic cyclone and flotation column, it also has the characteristics of the small lower limit of the effective flotation size, short separation time, large treating capacity, and low operation cost. 3. Experimental 3.1. Wastewater sample The wastewater sample for the experiment is taken from the crude oil, gas and water separation tank in the No. 6 joint Station of Shengli Gudao Oilfield. The oily wastewater sample is processed with the water quality analysis (Table 1). It can be seen that the
Table 1 Water quality analysis result of the oily wastewater in No. 6 joint Station of Shengli Gudao Oilfield. Type Color pH Temperature (°C) Density (g/cm3) Viscosity (mPa s) Oil content (mg/L) HPAM content (mg/L) Suspended solid content (mg/L)
Analysis result Non-settled is yellowish-brown, with lots of dispersed oil and suspended matters 7.01–7.45 35.00–39.00 910.00–960 1.5238 2000–2500 150–300 500–680
The experimental system (Fig. 2) includes the flotation column separation system and measurement control system. The separation system is composed of the cyclonic-static microbubble flotation column, feeding pump (for inflow and outflow), circulating pump, mixing tank (dosing and mixing), and other devices. The cyclonic-static microbubble flotation column for experiment was made of plexiglass with a diameter of 400 mm and a height of 4000 mm. The control system for measurement is composed of the gas flowmeter, liquid flowmeter, electric control valve, Pid digital regulator, gas content determinator, etc. The automatic control system for the flotation column liquid level is composed of the pressure transmitter, electronic control valve for straight travel, Pid digital regulator, etc. First of all, the oily sewage from the primary separation tank entered into the mixing drum and its flow was regulated; after that, the inflow was supplemented from the mid-upper part of the flotation column via the feeding pump; finally, the outflow was drained from the flotation column bottom via the control valve. The aeration rate will be measured and regulated by the gas flowmeter, and the circulating pressure was regulated by changing the circulating pump speed. 3.3. Methods In the experiment, the oil removal efficiency was used to evaluate the oil–water separation efficiency of the cyclonic separation section of the cyclonic-static microbubble flotation column. Sampling will be made from the sampling point of the tangential feeding port as well as the sampling point of the purified underflow outlet (Fig. 1), and the oil concentration of these two points are c1 and c2. The separation efficiency R is:
R ¼ 1 c2 =c1 where c1 is the concentration of the oil in the tangential feeding port; and c2 the concentration of the oil in the underflow outlet. The parameters influencing the cyclonic separation effect mainly cover the feeding rate, circulating pressure, aeration rate, overflow split ratio, etc. And, the feeding rate was controlled by the flowmeter. The circulating pressure refers to the pressure at
Inflow Flowmeter
Oily foam Mixing
Inflow
Pressure Outflow
Circulating pump
Fig. 2. Schematic diagram of the flotation column system.
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f ¼ q2 =ðq1 þ q2 Þ
3.4. Flow field test The type SCD-22 two dimensional frequency shift laser velocimeter (manufactured by Beijing Feilijia Technology Service Co., Ltd.) was applied for the velocity field test of the cyclonic separation section of the flotation column. The free coordinate frame of LDV is the freedom displacement system, and the laser can be freely moved according to the measurement points. The flotation column was made of glass with a diameter of 50 mm, a height of 2000 mm. In order to mitigate the impact of bent wall of the flotation column on laser, the column is encased with a glass-made optical compensation box outside (Fig. 3). In order to enhance the reflected light intensity of the fluid in the column, the fluid is mixed with the polystyrene as the tracer. In order to ensure the precision of the measurement results, no gas was inputted during circulation, and test will be conducted in fresh water. The test was conducted from the tangential inlet, and four faces will be tested, with the number of faces 1, 2, 3, and 4. For the test face, four test points were taken radially, which are distributed on the radius of the cyclonic separation; for each point, one group was tested with an interval of 1 min; 300 points will be tested and average values of 10 groups were taken. Radially (horizontal direction of the passage 1), the central point and places with distance of 5, 10, 15, and 20 mm were sampled; axially (vertical direction of the passage 2), the places with distance
of 10, 20, 30, and 40 mm were taken. The test was arranged in a reticular form.
4. Results and discussion 4.1. Effect of the operating parameters on the separation efficiency 4.1.1. Effect of the feeding rate Experimental conditions: p = 0.26 MPa, qc = 20 m3/h, qg = 3.0 m3/h, f = 95%. The feeding rate impact on the cyclonic separation effect is shown in Fig. 4. As shown in Fig. 4, with the increasing feeding rate, the oil removal rate reduces. If the feeding rate is increased, the separation time will be short, and the collision probability of oil droplet and bubble will be lowered down, which will be adverse to oil–water separation and decrease the oil removal. The proper feeding rate determined by test is 1.5 m3/h. 4.1.2. Effect of the aeration rate Experimental conditions: gl = 1.5 m3/h, p = 0.26 MPa, qc = 20 m3/h, f = 95%. The aeration rate impact on the cyclonic separation effect is shown in Fig. 5. As shown in Fig. 5, with the increasing aeration rate, the oil removal of the cyclonic separation section increases and become stable along with the increasing aeration rate. The aeration rate determined by test is 2.5 m3/h. 4.1.3. Effect of the circulating pressure Experimental conditions: gl = 1.5 m3/h, qg = 2.5 m3/h, f = 95%. The circulating pressure impact on the cyclonic separation effect is shown in Fig. 6.
88 86
Oil removal (%)
the circulating pump outlet of the flotation column; by controlling the pressure size, the tangential feeding velocity can be controlled to provide the cyclonic separation with different energies. In the experiment, the frequency converter (0–50 Hz) was used to change the circulating pump velocity and regulate its outlet pressure. The aeration rate refers to the amount of the gas sucked into the flotation column via the bubble generator, which can be measured and regulated via the gas flowmeter. After being separated in the flotation column, one part of oily foam in the oily wastewater will be drained out via the overflow outlet, and the remaining part will be drained out via the underflow outlet. The flow is expressed by q1 and q2. Furthermore, the split ratio is used to describe relations between the overflow outlet flow q1 and underflow outlet flow q2; the ratio between the oil removal equipment and underflow outlet flow and the total feeding amount is called as the underflow split ratio, that is:
84 82 80 78 76 74
0.5
1.0
1.5 2.0 q1 (m-3/h)
2.5
3.0
Fig. 4. Feeding rate impact on the oil removal by cyclonic separation.
90
Oil removal (%)
88 86 84 82 80 78 76 74
Fig. 3. Actual measurement by LDV.
0
1
2
qg (m-3/h)
3
4
Fig. 5. Aeration rate impact on the oil removal by cyclonic separation.
J. Liu et al. / International Journal of Mining Science and Technology 23 (2013) 415–422
Oil removal (%)
418
85
4.2. Mathematical model of cyclonic separation
80
The operating parameters including the feeding rate gl, aeration rate qg, circulating pressure p, and underflow split ratio f are main factors influencing the cyclonic separation effect. By analysing these operating parameters and cyclonic separation efficiency (namely the influence relation result of the oil removal R), it is found out that the cyclonic separation efficiency R has the following relations with the feeding rate gl, aeration rate qg, circulating pressure p, and underflow split ratio f, namely the cyclonic separation model:
75 70 65 60 0.10
0.15
0.20
0.25
0.30
0.35
p (MPa)
R ¼ 6:2356 0:0385p0:05413 þ 0:0295qg þ
0:02161
1:1855
4:4356f 0:3986 þ 3:6250q1:4617 l
Fig. 6. Circulating pressure impact on the oil removal by cyclonic separation.
As shown in Fig. 6, with the increasing circulating pressure, the oil removal by cyclonic separation gradually increases. When the circulating pressure reaches certain value, the oil removal will reach a maximum. Increasing circulating pressure the oil removal began to decrease. In a proper pressure range, smaller bubble sizes and more quantities will be generated in case of higher pressure. In addition, by collision between oil droplet and bubble, a proper energy will be offered, which will facilitate the oil–gas complex formation. When the circulating pressure is 0.30 MPa, the oil removal can reach to 83.25%. However, when the circulating pressure continues increasing with the power frequency, the centrifugal intensity becomes larger. Besides the oil droplet is under larger shearing force, small oil droplets are more easily formed and the cyclonic separation effect is adversely impacted. Moreover, the turbulence will be formed in the flotation column due to too large pressure strongly disturb the fluid pattern inside. Consequently the oil–gas complex is damaged and the separation effect will be decreased as well. The circulating pressure determined by the test is 0.28 MPa. 4.1.4. Effect of the underflow split ratio Experimental conditions: gl = 1.5 m3/h, p = 0.26 MPa, qc = 23 m3/h, qg = 2.5 m3/h. The underflow split ratio impact on the cyclonic separation effect is shown in Fig. 7. As shown in Fig. 7, with the decreasing underflow split ratio, the oil removal of the cyclonic separation section gradually increases and then decreases sharply. In order to increase the purified outflow of the underflow outlet, it is necessary to reduce water content in the oily overflow as well as solve problems arising from secondary purification. The determined underflow split ratio is 95%.
88
Oil removal (%)
87
The cyclonic separation model shows that the cyclonic separation efficiency, namely, the oil removal increases and then decreases along with the increasing circulating pressure p and aeration rate qg, and decreases and then increases along with the increasing feeding rate gl and underflow split ratio f. Fig. 8a–f mean the three dimensional response surface between the oil removal by cyclonic separation with different operating parameters. Fig. 8a shows the interaction impact of the feeding rate and underflow split ratio on the oil removal. If the feeding rate is determined, the oil removal decreases along with the increasing underflow split ratio. If the underflow split ratio is determined, the oil removal decreases along with the increasing feeding rate. Fig. 8b shows the interaction impact of the circulating pressure and underflow split ratio on the oil removal. If the circulating pressure level is determined, the oil removal decreases along with the increasing underflow split ratio. If the underflow split ratio is determined, the oil removal decreases along with the increasing circulating pressure. Fig. 8c shows the interaction impact of the aeration rate and underflow split ratio on the oil removal. If the aeration rate level is determined, the oil removal decreases along with the increasing underflow split ratio. If the underflow split ratio is determined, the oil removal increases along with the increasing aeration rate. Fig. 8d shows the interaction impact of the feeding rate and circulating pressure on the oil removal. If the circulating pressure level is determined, the oil removal decreases along with the increasing feeding rate. If the feeding rate is determined, the oil removal increases along with the increasing circulating pressure. Fig. 8e shows the interaction impact of the feeding rate and aeration rate on the oil removal. If the aeration rate level is determined, the oil removal decreases along with the increasing feeding rate. If the feeding rate level is determined, the oil removal increases along with the increasing aeration rate. Fig. 8f shows the interaction impact of the circulating pressure and aeration rate on the oil removal. If the aeration rate level is determined, the oil removal increases along with the increasing circulating pressure. If the circulating pressure level is determined, the oil removal increases along with the increasing aeration. 4.3. Cyclone velocity field test and simulation
86 85 84 83 82
80
85
90 f (%)
95
100
Fig. 7. Underflow split ratio impact on the oil removal by cyclonic separation.
4.3.1. Cyclone velocity field test The cyclone velocity field distribution rule under the circulating pressures of 0.04, 0.08, and 0.12 MPa was obtained (feeding rate of 770 mL/min and discharging speed of 540 mL/min). The test results are shown in Fig. 9. Axial velocity distribution rule: as shown in Fig. 9a, d and g, within the range of places with distance of 5–20 mm between the central point, the speed increases along with the increasing distance, and the axial velocity measurement range is 0–0.7 m/s; at the same radius distance, it decreases along with the increasing
419
0.4 95 Un 0.3 re ss u der 90 p re 0.2 g rat flow 85 t in ) io( sp %) lit 80 0.1 i rcul a (MPa C
100 90 80 70 60 50 3 0.35 2 Fee 0.25 0.3 di n 1 0.2 gv 0.15 s s ure (m -3 elo ci 0 0.1 g pre ty la tin ) ·h ) u c r i C (M Pa
88 86 84 82 80 78 100 4 Un 95 3 90 der -3 ) 2 flo ·h 1 rat i w s 85 80 t e (m 0 o( ion r a %) pl it Infl at
(b) Oil removal efficiency (%)
Oil removal efficiency (%)
(a)
Oil removal efficiency (%)
100 90 80 70 60 100
95 90 85 80 75 2.5 2.0 4 Fe 3 3 edin 1.5 - ·h ) 2 1.0 gv m 1 0.5 0 (m -3 elo c a te ( ity ti on r ·h) Infl a
(d)
(e)
(c) Oil removal efficiency (%)
92 90 88 86 84 82 80 100 0.5 1.0 95 90 Fee di 1.5 2.0 t i l 85 p s ng ve rfl ow locit y 2.5 80 Unde io ( %) ( m -3· h) ra t
Oil removal efficiency (%)
Oil removal efficiency (%)
J. Liu et al. / International Journal of Mining Science and Technology 23 (2013) 415–422
100 90 80 70 60 50 4 Fee 3 0.3 0.35 d in 2 gv 1 0.2 0.25 0.15 e sure (m -3 l oc 0 0.1 g pre s · h) i ty l at in ) u c r i C ( MP a
(f)
Fig. 8. Three dimensional response surface between the oil removal efficiency by cyclonic separation and operating parameters.
distance between the tangential inlet. With the increasing circulating pressure and circulating tangential feeding rate, the axial velocity distribution rule almost keep the same; however, the axial velocity on all sections has an increasing trend. Radial velocity distribution rule: as shown in Fig. 9b, e and h, within the range of places with distance of 5–20 mm between the central points, the velocity increases along with the increasing distance, and the radial velocity measurement range is 0–0.3 m/s; at the same radius distance, it decreases along with the increasing distance between the tangential inlet. With the increasing circulating pressure and circulating tangential feeding rate, the radial velocity distribution rule almost keep the same; however, the radial velocity on all sections has an increasing trend. Resultant velocity distribution rule: as shown in Fig. 9c, f and i, within the range of places with distance of 5–20 mm between the central point, the velocity decreases along with the increasing distance, and the radial velocity measurement range is 0–0.03 m/s. 4.3.2. Cyclone velocity field simulation The numerical simulation was applied with the standard k–e model, and the Fluent software was applied for numerical simulation. The lattice division is shown in Fig. 10, and the selected boundary conditions and parameters are shown in Table 2. Axial velocity distribution rule: Fig. 11a shows the axial velocity distribution rule on each section of cyclone sections with different circulation volumes. The axial velocity is mostly negative value and the velocity direction is downward. With the increase of the circulating pressure and the circulating tangential feeding rate, the axial velocity distribution rule is essentially unchanged, but the axial velocity of each section has an increasing trend. Radial velocity distribution rule: Fig. 11b shows the radial velocity distribution rule on each section of cyclone sections with
different circulation volumes. The radial velocity basically presents as the bilaterally symmetrical distribution, with one half as positive value and the other half as negative value. It means that the direction of radial velocity is from the column wall to the axle center. With the increase of circulating pressure and the circulating tangential feeding rate, the radial velocity distribution rule is essentially unchanged, but the radial velocity of each section has an increasing trend. Tangential velocity distribution rule: Fig. 11c shows the tangential velocity distribution rule on each section of cyclone sections with different circulation volumes. The tangential velocity distribution basically presents as the symmetrical distribution, the value of negative half axis at the side of tangential inlet exceeds the value of positive half axis at the other side; from the column wall to the center point, the tangent velocity rapidly increases to the maximum at first and then slowly decreases to 0 at the axle center. With the increase of circulating pressure and the circulating tangential feeding rate, the tangential velocity distribution rule is essentially unchanged, but the tangential velocity of each section has an increasing trend, and the cyclone intensity is enhanced. Test face 3 and the analog section Z = 0.28 are the sections at the same position on the flotation column. Due to the limitation of Doppler Laser Velocimeter the axial and radial velocity of face 3 can only be acquired by testing instrument. The tangential velocity cannot be measured. The study conducts the comparative analysis to the test and analog result of the axial and radial velocity when the circulating pressure is 0.08 MPa and the circulation volume is 0.324 m3/h. The radial and axial velocity of the selected test face 3 when the rotational velocity of feed pump is 300 r/min, the rotational velocity of discharge pump is 197 r/min, and the circulating pressure is 0.08 MPa.
J. Liu et al. / International Journal of Mining Science and Technology 23 (2013) 415–422
Face Face Face Face
0.022 0.020 0.018
1 2 3 4
0.016 0.014
V (m/s)
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05
Vr (m/s)
Vz (m/s)
420
0.012 0.010
10
15
20
5
10
10
15
20
5
10
15
20
20
10
R (mm)
(g) p=0.12 MPa axial velocity distribution
20
5
10
15
20
(f) p=0.08 MPa resultant velocity distribution
V (m/s)
0.030 0.028 0.026 0.024 0.022 0.020 0.018 0.016 0.014 5
15
R (mm)
(e) p=0.08 MPa radial velocity distribution
Vr (m/s)
Vz (m/s)
15
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05
R (mm)
(d) p=0.08 MPa axial velocity distribution
10
10
(c) p=0.04 MPa resultant velocity distribution
V (m/s)
0.024 0.022 0.020 0.018 0.016 0.014 0.012 0.010 0.008 0.006
R (mm)
0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 5
20
R (mm)
(b) p=0.04 MPa radial velocity distribution
Vr (m/s)
Vz (m/s)
(a) p=0.04 MPa axial velocity distribution
5
15 R (mm)
R (mm)
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05
0.20 0.15 0.10 0.05 5
0.008 0.006 5
0.45 0.40 0.35 0.30 0.25
15
20
0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 5
10
R (mm)
(h) p=0.12 MPa radial velocity distribution
15
20
R (mm)
(i) p=0.12 MPa resultant velocity distribution
Fig. 9. Test results by LDV.
Z= 0.31 face Z=0.28 face Z=0.25 face Z=0.22 face Z= 0.19 face Y
Z X
eter and the influence of boundary effect, the error of the measured results near the column axis and column wall is too large, and without regularity. Therefore, it is not indicated on the test result. It can be explained as that the test and the analog axial velocity distributions are basically the same. According to the comparison between Fig. 8e. Fig. 11b, we can find that: the radial velocity range of test result is 0–0.02 m/s, the radial velocity range of analog result is 0–0.05 m/s; from the center point to the position with distance of 0.013 m to the center Table 2 Boundary conditions and parameters for simulation.
Fig. 10. Lattice division.
Item Structure parameter
According to the comparison between Fig. 8d. Fig. 11a, we can find that: the axial velocity range of test result is 0–0.45 m/s, and the axial velocity range of analog result is 0–0.35 m/s; from the position with distance of 0.015 m to the axle center to the position with distance of 0.020 m to the axle center, the test and the analog axial velocity present the tendency of gradually increasing to the maximum; from the position with distance of 0.20 m to the column side wall, the analog axial velocity decreases rapidly to 0 from the maximum. Because of the limitation of Doppler Laser Velocim-
Water-phase property parameter
Parameter Height (m) Diameter (mm) Feeding rate (m/s) Discharging rate (m/s) Circulating pressure (MPa) Tangential feeding rate (m/s)
2 50 0.006539 0.11465 0.08
Density (kg/m3) Dynamic viscosity (Pa s)
998.23 1.005 103
0.796178
The velocity distribution rules of the cyclone section are shown in Fig. 11.
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4.00e-01
2.00e-01 1.00e-01
Radial velocity (m/s)
Axial velocity (m/s)
2.00e-01
Line 0.19 Line 0.22 Line 0.25 Line 0.28 Line 0.31
3.00e-01
0 -1.00e-01 -2.00e-01 -3.00e-01 -0.025
-0.015
-0.005 0.005 Position (m)
0.015
1.50e-01 1.00e-01 5.00e-02 0 -5.00e-02 -1.00e-01 -0.025
0.025
Tangential velocity (m/s)
2.00e-01 1.00e-01 0 -1.00e-01 -2.00e-01 -3.00e-01 -4.00e-01 -0.025
-0.015
-0.005 0.005 Position (m)
0.015
0.025
(c) Tangential velocity distribution of all sections
-0.005 0.005 Position (m)
0.015
0.025
(b) Radial velocity distribution of all sections
Relative velocity magnitude (m/s)
(a) Axial velocity distribution of all sections
-0.015
5.00e-01 4.50e-01 4.00e-01 3.50e-01 3.00e-01 2.50e-01 2.00e-01 1.50e-01 1.00e-01 5.00e-02 0 -0.025
-0.015
-0.005 0.005 Position (m)
0.015
0.025
(d) Relative resultant velocity distribution of all sections
Fig. 11. Velocity distribution rules of the cyclone section.
point, the analog axial velocity gradually increases from 0 to the maximum; from the position with distance of 0.013 m to the center point to the position with distance of about 0.020 m to the center point, both of the test and analog axial velocity present the tendency of gradual decrease; from the position with distance of 0.20 m to the center point to the column side wall, the analog axial velocity continuously and gradually decreases to 0. Due to the limitation of doppler laser velocimeter and the influence of boundary effect, the error of the measured results near the column axis and column wall is too large and without regularity. Therefore, it is not indicated on the test result. It can be explained as that the test and the analog axial velocity distributions are basically the same.
wastewater discharged at the overflow weir will cause oily concentration being lower, the quantity of wastewater for further treating will be increased, and so as the treatment cost. Therefore, a less volume of this part is better. The test determines the best operation condition: the feeding rate is 1.50 m3/h, the aeration rate is 2.50 m3/h, the circulating pressure is 0.28 MPa, and the underflow split ratio is 95%. (2) The relation between the cyclone separation efficiency R and the feeding rate gl, the aeration rate qg, the circulating pressure p, the underflow split ratio f is described as the following, which is also called the cyclone separation model:
R ¼ 6:2356 0:0385p0:05413 þ 0:0295qg
5. Conclusions
þ (1) Increasing the feeding rate, the oil removal will gradually decrease, the appropriate aeration rate is important for improving the cyclonic separation efficiency. With the increasing aeration rate to a certain value, the bubbles coalescence and fluid turbulence occur violently. The bubbles with smaller diameter, which are more capable for capturing fine oil droplets, gradually decrease and result in the oil removal decreasing. The circulating pressure is required to be set within an appropriate range. If the circulating pressure is low the centrifugal intensity will not be enough and the cyclone separation effect will be poor. While the pressure is too high, the oil droplets with larger diameter will be easily cut into small droplets, and the higher turbulence intensity of the cyclone separation section will cause low cyclone separation efficiency. Meeting the condition that the oil concentration of cleaning water discharged at the bottom outlet is a regulated content, and the larger of underflow split ratio is better. A bigger volume of oily
0:02161 þ 3:6250q1:4617 l
1:1855
4:4356f 0:3986
(3) Adopting the laser velocimeter, the axial velocity distribution rule of cyclone separation section was measured: within the range between the positions with distances of 0.005 and 0.02 m to the center point, the velocity increases with the increasing distance to the center point, the measuring range of axial velocity is 0–0.7 m/s; the radial velocity distribution rule: within the range between positions with distances of 0.005 and 0.02 m to the center point, the velocity decreases with the increasing distance to the center point, the measuring range of radial velocity is 0–0.03 m/s. (4) The axial velocity distribution rule of cyclone separation section is acquired by the cyclone numerical simulation results: the axial velocity presents the tendency of decline when going upward or downward along the tangent feeding surface Z = 0.25; generally speaking, the axial velocity are mostly negative value, and the velocity direction is downward along the axis; the radial velocity distribution
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rule: the radial velocity is less than the axial velocity; from the column wall to the axis center, the radial velocity distribution rule is that rapidly increasing at first and then decreasing to 0; the radial velocity direction is from the column wall to the axle center; the tangential velocity distribution rule: the tangential velocity is larger than the axial velocity and the radial velocity; it basically presents as the symmetrical distribution; the value of negative half axis at the side of tangential inlet is larger than the value of positive half axis at the other side; from the column wall to the center point, the tangent velocity rapidly increases to the maximum at first, and then slowly decreases to 0 at the axis center. According to the comparative analysis conducted to the test and analog results by applying the two velocity components of axial velocity and radial velocity, we know that the regularities of the analog and test results are basically the same, which verifies the reliability of the numerical simulation. Acknowledgment The authors are grateful to the National Natural Science Foundation of China (No. 50974119) for the financial support for this project. References [1] Emamjomeh MM, Sivakumar M. Review of pollutants removed by electrocoagulation and electrocoagulation/flotation processes. J Environ Manage 2009;90(5):1663–79. [2] Li XB, Liu JT, Wang YT, Wang CY, Zhou XH. Separation of oil from wastewater by column flotation. J China Univ Min Technol 2007;17(4):546–51. 577. [3] Capponi F, Sartori M, Souza ML, Rubio J. Modified column flotation of adsorbing iron hydroxide colloidal precipitates. Int J Miner Process 2006;79(3):167–73.
[4] Salah AT, Yoon RH, Shin D. A comparison of anionic and cationic flotation of a siliceous phosphate rock in a column flotation cell. Min Sci Technol 2011;21(1):147–51. [5] Zhang Y, Wang Y, Li S. Flotation separation of calcareous minerals using didodecyldimethylammonium chloride as a collector. Int J Min Sci Technol 2012;22(2):285–8. [6] Zhu H, Zhang G, Liu G, Sun Q. Flotation bubble seed image filling algorithm based on boundary point features. Int J Min Sci Technol 2012;22(3): 289–93. [7] Sun W, Chen C, Tang H. Effects and action mechanism of CO32-on flotation rate of calcite. J China Univ Min Technol 2012;41(6):48–51. [8] Liao Y, Liu J, Wang Y, Cao Y. Prediction of gas holdup in cyclonic static micro bubble flotation column based on BP neural networks. J China Univ Min Technol 2011;40(3):443–7. [9] Peng ZX. Application of flotation machine in oilfield oil sewage treatment. Petrol Plann Eng 1994;5(3):49–50. [10] Wang TS. Status and prospects for oilfield water treatment equipment. Petrol Mach 1999;27(7):l–4. [11] Zheng YY. Mathematical model and technological design of suction flotation purifier. J China Univ Petrol 1990;14(2):68–73. [12] Bennett GF. The removal of oil from wastewater by air flotation: a review. CRC Crit Rev Environ Control 1988;18(3):189. [13] Gao GC. A new type of oily sewage treatment equipment-Jet flotation device. China Offshore Oil Gas 2000;3:57–9. [14] Du YB. Development of jet flotation device. Oil-Gasfield Surf Eng 1999;18(4):32–5. [15] Feris LA, Rubio J, Schneider IH. Remocao decorantes emefluentes dotingimento deagatas porflotac taode partculas adsorventes. In: Proceeding of 19th Congresso Brasileiro de Engenharia Sanit aria e Ambiental, Rio de Janeiro; 1999. [16] Huang F. Research on structure design and flow field numerical simulation for twin-jet flotation column. Beijing: China University of Mining and Technology; 1998. [17] Ye Y, Gopalakrishnan S, Pacquet E, Miller JD. Development of the air-sparged hydrocyclone – a swirl flotation column. In: Proceedings of international column flotation symposium. Phoenix, AZ: AIME/SME; 1988. p. 305. [18] Zhi TX. Research situation and application prospect of flotation column. China Coal 1999;25(6):56–61. [19] Liu JT. Study on cyclonic-static micro bubble flotation column and preparation technology of clean coal. Beijing: China University of Mining and Technology; 1998. [20] Tang HX, Qian Y, Wen XH. Characteristics and control technology principle of water particles and nonbiodegradable organics. Beijing: China Environmental Science Press; 2000.
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Cyclonic separation process intensification oil removal based on microbubble flotation Liu Jiongtian ⇑, Xu Hongxiang, Li Xiaobing Chinese 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
Article history: Received 2 October 2012 Received in revised form 25 October 2012 Accepted 10 November 2012 Available online 9 July 2013 Keywords: Cyclonic-static microbubble flotation column Microbubble flotation Cyclonic separation Oil–water separation
a b s t r a c t The cyclonic-static microbubble flotation column has dual effects including the cyclonic separation and floatation separation with the characteristics of the small lower limit of the effective separation size, short separation time, large handling capacity, and low operation cost. It shows significant advantages in the oily wastewater treatment field, especially the polymer flooding oily wastewater treatment aspect. In this paper, the cyclonic separation function mechanism of the cyclonic-static microbubble flotation column was studied, the impact of the parameters including the feeding rate, aeration rate, circulating pressure, and underflow split ratio on the cyclonic separation efficiency was investigated, and the cyclonic separation efficiency model was established as well. In addition, by applying the Doppler Laser Velocimeter (LDV) and Fluent simulation software, the test and simulation to the single-phase flow velocity field of the cyclonic separation section of the cyclonic-static microbubble flotation column were carried out, and the velocity distribution rule of the cyclonic separation section was analyzed under the singlephase flow conditions. Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Currently, many domestic oilfields have stepped into the medium and high water cut stage of the mid-to-late petroleum exploitation period; thus, the oily wastewater treatment has become one of the key challenges for the oilfield mine production and environment protection. Initially, the floatation is used to selectively separate the targeted mineral from the non-targeted mineral by foam. With advantages of high separation efficiency, large treating capacity and low cost, the flotation technology has become a kind of water treatment technology widely researched both at home and abroad, and has been gaining more and more popularity [1–8]. With wider application of the flotation technology in the oily wastewater treatment, many sets of flotation equipment have emerged correspondingly. The common flotation equipment covers the impeller suction floatation machine, air jet flotation machine, eccentric flotation machine, pneumatic flotation column, pneumatic cyclone, etc. [9–18]. Though such kinds of flotation equipment can show superior functions in pre-treatment of the water flooding liquid and wastewater treatment of the oilfield, bubbles arising from entrainment and back mixing as well as slow oil drop coalescence due to poor contact environment and smaller application range of the oil concentration for raw water are main parameters influencing the separation efficiency. In addition, with increased liquid production by
polymer flooding and ternary complex, the application of the common flotation equipment is also seriously restricted. Under this circumstance, researching and developing the efficient flotation equipment is important to the oilfield development. A kind of cyclonic-static microbubble flotation column with unique structures has been introduced to the oil–water separation field [19]. The cyclonic-static microbubble flotation column has also been applied to make the industrial test in Shengli Gudao Oilfield. The oil content of the crude oil, gas and water separation tank in the joint Station of Gudao Oilfield is 2000–2500 mg/L, which has been reduced to below 10 mg/L after being subject to the separation treatment by the flotation column. The cyclonic-static microbubble flotation column is a complicated separation process integrated with the common cyclonic separation and common foam flotation separation. The built-in cyclone of the cyclonic-static microbubble flotation column is a kind of unique gas-injected liquid–liquid hydraulic cyclone; on this account, the research is based on the forced oil removal process by cyclonic separation of the microbubble flotation; the oil–water separation efficiency of the cyclonic separation section of the cyclonic-static microbubble flotation column is significant to further improve the oil–water separation efficiency of the cyclonic-static microbubble flotation column. 2. Discussion of the cyclonic separation process
⇑ Corresponding author. Tel.: +86 516 83885878. E-mail address: [email protected] (J. Liu).
The cyclonic-static microbubble flotation column (Fig. 1) is integrated with the floatation column on the basis of the common
2095-2686/$ - see front matter Ó 2013 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.05.010
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Airflotation
oily wastewater quality of the No. 6 joint Station of Shengli Gudao Oilfield is poor; the non-settled is yellowish-brown, with logs of dispersed oil and suspended matter. Besides, due to high oil content and high content of suspended matters and polymers, the emulsion in the oily wastewater of the No. 6 joint Station of Shengli Gudao Oilfield was stable, which is too difficult to treatment.
Cyclonic
3.2. Experimental set-up
Fig. 1. Cyclonic-static microbubble flotation column.
static hydraulic cyclone, and the oil–gas complex which is generated by carrier effect of the self-suction bubble generator of the flotation column will transport the emulsified and fine-graded oil droplets in the oily wastewater to the column separation area from the cyclonic separation area; then by applying the static separation effect of the ‘‘long and narrow’’ environment with ‘‘quiet’’ fluid dynamics in the column separation area, the oil–water separation will be carried out [20]. Consequently, after being pressurized by the circulating pump, the oily wastewater with poor separability will enter into the built-in cyclone of the flotation column from the middle and lower parts of the flotation column, and perform further cyclonic separation under the effect of the centrifugal force field. As the cyclonic-static microbubble flotation column is integrated with the cyclonic separation technology and flotation separation technology, it has dual effects including the cyclonic separation and airfloatation separation. In addition, the synergistic effect of multiple separation modes reinforces the separation effect and expands the size range of the oil content in the oily wastewater for flotation separation. In addition to a higher treating efficiency, when being compared to the common hydraulic cyclone and flotation column, it also has the characteristics of the small lower limit of the effective flotation size, short separation time, large treating capacity, and low operation cost. 3. Experimental 3.1. Wastewater sample The wastewater sample for the experiment is taken from the crude oil, gas and water separation tank in the No. 6 joint Station of Shengli Gudao Oilfield. The oily wastewater sample is processed with the water quality analysis (Table 1). It can be seen that the
Table 1 Water quality analysis result of the oily wastewater in No. 6 joint Station of Shengli Gudao Oilfield. Type Color pH Temperature (°C) Density (g/cm3) Viscosity (mPa s) Oil content (mg/L) HPAM content (mg/L) Suspended solid content (mg/L)
Analysis result Non-settled is yellowish-brown, with lots of dispersed oil and suspended matters 7.01–7.45 35.00–39.00 910.00–960 1.5238 2000–2500 150–300 500–680
The experimental system (Fig. 2) includes the flotation column separation system and measurement control system. The separation system is composed of the cyclonic-static microbubble flotation column, feeding pump (for inflow and outflow), circulating pump, mixing tank (dosing and mixing), and other devices. The cyclonic-static microbubble flotation column for experiment was made of plexiglass with a diameter of 400 mm and a height of 4000 mm. The control system for measurement is composed of the gas flowmeter, liquid flowmeter, electric control valve, Pid digital regulator, gas content determinator, etc. The automatic control system for the flotation column liquid level is composed of the pressure transmitter, electronic control valve for straight travel, Pid digital regulator, etc. First of all, the oily sewage from the primary separation tank entered into the mixing drum and its flow was regulated; after that, the inflow was supplemented from the mid-upper part of the flotation column via the feeding pump; finally, the outflow was drained from the flotation column bottom via the control valve. The aeration rate will be measured and regulated by the gas flowmeter, and the circulating pressure was regulated by changing the circulating pump speed. 3.3. Methods In the experiment, the oil removal efficiency was used to evaluate the oil–water separation efficiency of the cyclonic separation section of the cyclonic-static microbubble flotation column. Sampling will be made from the sampling point of the tangential feeding port as well as the sampling point of the purified underflow outlet (Fig. 1), and the oil concentration of these two points are c1 and c2. The separation efficiency R is:
R ¼ 1 c2 =c1 where c1 is the concentration of the oil in the tangential feeding port; and c2 the concentration of the oil in the underflow outlet. The parameters influencing the cyclonic separation effect mainly cover the feeding rate, circulating pressure, aeration rate, overflow split ratio, etc. And, the feeding rate was controlled by the flowmeter. The circulating pressure refers to the pressure at
Inflow Flowmeter
Oily foam Mixing
Inflow
Pressure Outflow
Circulating pump
Fig. 2. Schematic diagram of the flotation column system.
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f ¼ q2 =ðq1 þ q2 Þ
3.4. Flow field test The type SCD-22 two dimensional frequency shift laser velocimeter (manufactured by Beijing Feilijia Technology Service Co., Ltd.) was applied for the velocity field test of the cyclonic separation section of the flotation column. The free coordinate frame of LDV is the freedom displacement system, and the laser can be freely moved according to the measurement points. The flotation column was made of glass with a diameter of 50 mm, a height of 2000 mm. In order to mitigate the impact of bent wall of the flotation column on laser, the column is encased with a glass-made optical compensation box outside (Fig. 3). In order to enhance the reflected light intensity of the fluid in the column, the fluid is mixed with the polystyrene as the tracer. In order to ensure the precision of the measurement results, no gas was inputted during circulation, and test will be conducted in fresh water. The test was conducted from the tangential inlet, and four faces will be tested, with the number of faces 1, 2, 3, and 4. For the test face, four test points were taken radially, which are distributed on the radius of the cyclonic separation; for each point, one group was tested with an interval of 1 min; 300 points will be tested and average values of 10 groups were taken. Radially (horizontal direction of the passage 1), the central point and places with distance of 5, 10, 15, and 20 mm were sampled; axially (vertical direction of the passage 2), the places with distance
of 10, 20, 30, and 40 mm were taken. The test was arranged in a reticular form.
4. Results and discussion 4.1. Effect of the operating parameters on the separation efficiency 4.1.1. Effect of the feeding rate Experimental conditions: p = 0.26 MPa, qc = 20 m3/h, qg = 3.0 m3/h, f = 95%. The feeding rate impact on the cyclonic separation effect is shown in Fig. 4. As shown in Fig. 4, with the increasing feeding rate, the oil removal rate reduces. If the feeding rate is increased, the separation time will be short, and the collision probability of oil droplet and bubble will be lowered down, which will be adverse to oil–water separation and decrease the oil removal. The proper feeding rate determined by test is 1.5 m3/h. 4.1.2. Effect of the aeration rate Experimental conditions: gl = 1.5 m3/h, p = 0.26 MPa, qc = 20 m3/h, f = 95%. The aeration rate impact on the cyclonic separation effect is shown in Fig. 5. As shown in Fig. 5, with the increasing aeration rate, the oil removal of the cyclonic separation section increases and become stable along with the increasing aeration rate. The aeration rate determined by test is 2.5 m3/h. 4.1.3. Effect of the circulating pressure Experimental conditions: gl = 1.5 m3/h, qg = 2.5 m3/h, f = 95%. The circulating pressure impact on the cyclonic separation effect is shown in Fig. 6.
88 86
Oil removal (%)
the circulating pump outlet of the flotation column; by controlling the pressure size, the tangential feeding velocity can be controlled to provide the cyclonic separation with different energies. In the experiment, the frequency converter (0–50 Hz) was used to change the circulating pump velocity and regulate its outlet pressure. The aeration rate refers to the amount of the gas sucked into the flotation column via the bubble generator, which can be measured and regulated via the gas flowmeter. After being separated in the flotation column, one part of oily foam in the oily wastewater will be drained out via the overflow outlet, and the remaining part will be drained out via the underflow outlet. The flow is expressed by q1 and q2. Furthermore, the split ratio is used to describe relations between the overflow outlet flow q1 and underflow outlet flow q2; the ratio between the oil removal equipment and underflow outlet flow and the total feeding amount is called as the underflow split ratio, that is:
84 82 80 78 76 74
0.5
1.0
1.5 2.0 q1 (m-3/h)
2.5
3.0
Fig. 4. Feeding rate impact on the oil removal by cyclonic separation.
90
Oil removal (%)
88 86 84 82 80 78 76 74
Fig. 3. Actual measurement by LDV.
0
1
2
qg (m-3/h)
3
4
Fig. 5. Aeration rate impact on the oil removal by cyclonic separation.
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Oil removal (%)
418
85
4.2. Mathematical model of cyclonic separation
80
The operating parameters including the feeding rate gl, aeration rate qg, circulating pressure p, and underflow split ratio f are main factors influencing the cyclonic separation effect. By analysing these operating parameters and cyclonic separation efficiency (namely the influence relation result of the oil removal R), it is found out that the cyclonic separation efficiency R has the following relations with the feeding rate gl, aeration rate qg, circulating pressure p, and underflow split ratio f, namely the cyclonic separation model:
75 70 65 60 0.10
0.15
0.20
0.25
0.30
0.35
p (MPa)
R ¼ 6:2356 0:0385p0:05413 þ 0:0295qg þ
0:02161
1:1855
4:4356f 0:3986 þ 3:6250q1:4617 l
Fig. 6. Circulating pressure impact on the oil removal by cyclonic separation.
As shown in Fig. 6, with the increasing circulating pressure, the oil removal by cyclonic separation gradually increases. When the circulating pressure reaches certain value, the oil removal will reach a maximum. Increasing circulating pressure the oil removal began to decrease. In a proper pressure range, smaller bubble sizes and more quantities will be generated in case of higher pressure. In addition, by collision between oil droplet and bubble, a proper energy will be offered, which will facilitate the oil–gas complex formation. When the circulating pressure is 0.30 MPa, the oil removal can reach to 83.25%. However, when the circulating pressure continues increasing with the power frequency, the centrifugal intensity becomes larger. Besides the oil droplet is under larger shearing force, small oil droplets are more easily formed and the cyclonic separation effect is adversely impacted. Moreover, the turbulence will be formed in the flotation column due to too large pressure strongly disturb the fluid pattern inside. Consequently the oil–gas complex is damaged and the separation effect will be decreased as well. The circulating pressure determined by the test is 0.28 MPa. 4.1.4. Effect of the underflow split ratio Experimental conditions: gl = 1.5 m3/h, p = 0.26 MPa, qc = 23 m3/h, qg = 2.5 m3/h. The underflow split ratio impact on the cyclonic separation effect is shown in Fig. 7. As shown in Fig. 7, with the decreasing underflow split ratio, the oil removal of the cyclonic separation section gradually increases and then decreases sharply. In order to increase the purified outflow of the underflow outlet, it is necessary to reduce water content in the oily overflow as well as solve problems arising from secondary purification. The determined underflow split ratio is 95%.
88
Oil removal (%)
87
The cyclonic separation model shows that the cyclonic separation efficiency, namely, the oil removal increases and then decreases along with the increasing circulating pressure p and aeration rate qg, and decreases and then increases along with the increasing feeding rate gl and underflow split ratio f. Fig. 8a–f mean the three dimensional response surface between the oil removal by cyclonic separation with different operating parameters. Fig. 8a shows the interaction impact of the feeding rate and underflow split ratio on the oil removal. If the feeding rate is determined, the oil removal decreases along with the increasing underflow split ratio. If the underflow split ratio is determined, the oil removal decreases along with the increasing feeding rate. Fig. 8b shows the interaction impact of the circulating pressure and underflow split ratio on the oil removal. If the circulating pressure level is determined, the oil removal decreases along with the increasing underflow split ratio. If the underflow split ratio is determined, the oil removal decreases along with the increasing circulating pressure. Fig. 8c shows the interaction impact of the aeration rate and underflow split ratio on the oil removal. If the aeration rate level is determined, the oil removal decreases along with the increasing underflow split ratio. If the underflow split ratio is determined, the oil removal increases along with the increasing aeration rate. Fig. 8d shows the interaction impact of the feeding rate and circulating pressure on the oil removal. If the circulating pressure level is determined, the oil removal decreases along with the increasing feeding rate. If the feeding rate is determined, the oil removal increases along with the increasing circulating pressure. Fig. 8e shows the interaction impact of the feeding rate and aeration rate on the oil removal. If the aeration rate level is determined, the oil removal decreases along with the increasing feeding rate. If the feeding rate level is determined, the oil removal increases along with the increasing aeration rate. Fig. 8f shows the interaction impact of the circulating pressure and aeration rate on the oil removal. If the aeration rate level is determined, the oil removal increases along with the increasing circulating pressure. If the circulating pressure level is determined, the oil removal increases along with the increasing aeration. 4.3. Cyclone velocity field test and simulation
86 85 84 83 82
80
85
90 f (%)
95
100
Fig. 7. Underflow split ratio impact on the oil removal by cyclonic separation.
4.3.1. Cyclone velocity field test The cyclone velocity field distribution rule under the circulating pressures of 0.04, 0.08, and 0.12 MPa was obtained (feeding rate of 770 mL/min and discharging speed of 540 mL/min). The test results are shown in Fig. 9. Axial velocity distribution rule: as shown in Fig. 9a, d and g, within the range of places with distance of 5–20 mm between the central point, the speed increases along with the increasing distance, and the axial velocity measurement range is 0–0.7 m/s; at the same radius distance, it decreases along with the increasing
419
0.4 95 Un 0.3 re ss u der 90 p re 0.2 g rat flow 85 t in ) io( sp %) lit 80 0.1 i rcul a (MPa C
100 90 80 70 60 50 3 0.35 2 Fee 0.25 0.3 di n 1 0.2 gv 0.15 s s ure (m -3 elo ci 0 0.1 g pre ty la tin ) ·h ) u c r i C (M Pa
88 86 84 82 80 78 100 4 Un 95 3 90 der -3 ) 2 flo ·h 1 rat i w s 85 80 t e (m 0 o( ion r a %) pl it Infl at
(b) Oil removal efficiency (%)
Oil removal efficiency (%)
(a)
Oil removal efficiency (%)
100 90 80 70 60 100
95 90 85 80 75 2.5 2.0 4 Fe 3 3 edin 1.5 - ·h ) 2 1.0 gv m 1 0.5 0 (m -3 elo c a te ( ity ti on r ·h) Infl a
(d)
(e)
(c) Oil removal efficiency (%)
92 90 88 86 84 82 80 100 0.5 1.0 95 90 Fee di 1.5 2.0 t i l 85 p s ng ve rfl ow locit y 2.5 80 Unde io ( %) ( m -3· h) ra t
Oil removal efficiency (%)
Oil removal efficiency (%)
J. Liu et al. / International Journal of Mining Science and Technology 23 (2013) 415–422
100 90 80 70 60 50 4 Fee 3 0.3 0.35 d in 2 gv 1 0.2 0.25 0.15 e sure (m -3 l oc 0 0.1 g pre s · h) i ty l at in ) u c r i C ( MP a
(f)
Fig. 8. Three dimensional response surface between the oil removal efficiency by cyclonic separation and operating parameters.
distance between the tangential inlet. With the increasing circulating pressure and circulating tangential feeding rate, the axial velocity distribution rule almost keep the same; however, the axial velocity on all sections has an increasing trend. Radial velocity distribution rule: as shown in Fig. 9b, e and h, within the range of places with distance of 5–20 mm between the central points, the velocity increases along with the increasing distance, and the radial velocity measurement range is 0–0.3 m/s; at the same radius distance, it decreases along with the increasing distance between the tangential inlet. With the increasing circulating pressure and circulating tangential feeding rate, the radial velocity distribution rule almost keep the same; however, the radial velocity on all sections has an increasing trend. Resultant velocity distribution rule: as shown in Fig. 9c, f and i, within the range of places with distance of 5–20 mm between the central point, the velocity decreases along with the increasing distance, and the radial velocity measurement range is 0–0.03 m/s. 4.3.2. Cyclone velocity field simulation The numerical simulation was applied with the standard k–e model, and the Fluent software was applied for numerical simulation. The lattice division is shown in Fig. 10, and the selected boundary conditions and parameters are shown in Table 2. Axial velocity distribution rule: Fig. 11a shows the axial velocity distribution rule on each section of cyclone sections with different circulation volumes. The axial velocity is mostly negative value and the velocity direction is downward. With the increase of the circulating pressure and the circulating tangential feeding rate, the axial velocity distribution rule is essentially unchanged, but the axial velocity of each section has an increasing trend. Radial velocity distribution rule: Fig. 11b shows the radial velocity distribution rule on each section of cyclone sections with
different circulation volumes. The radial velocity basically presents as the bilaterally symmetrical distribution, with one half as positive value and the other half as negative value. It means that the direction of radial velocity is from the column wall to the axle center. With the increase of circulating pressure and the circulating tangential feeding rate, the radial velocity distribution rule is essentially unchanged, but the radial velocity of each section has an increasing trend. Tangential velocity distribution rule: Fig. 11c shows the tangential velocity distribution rule on each section of cyclone sections with different circulation volumes. The tangential velocity distribution basically presents as the symmetrical distribution, the value of negative half axis at the side of tangential inlet exceeds the value of positive half axis at the other side; from the column wall to the center point, the tangent velocity rapidly increases to the maximum at first and then slowly decreases to 0 at the axle center. With the increase of circulating pressure and the circulating tangential feeding rate, the tangential velocity distribution rule is essentially unchanged, but the tangential velocity of each section has an increasing trend, and the cyclone intensity is enhanced. Test face 3 and the analog section Z = 0.28 are the sections at the same position on the flotation column. Due to the limitation of Doppler Laser Velocimeter the axial and radial velocity of face 3 can only be acquired by testing instrument. The tangential velocity cannot be measured. The study conducts the comparative analysis to the test and analog result of the axial and radial velocity when the circulating pressure is 0.08 MPa and the circulation volume is 0.324 m3/h. The radial and axial velocity of the selected test face 3 when the rotational velocity of feed pump is 300 r/min, the rotational velocity of discharge pump is 197 r/min, and the circulating pressure is 0.08 MPa.
J. Liu et al. / International Journal of Mining Science and Technology 23 (2013) 415–422
Face Face Face Face
0.022 0.020 0.018
1 2 3 4
0.016 0.014
V (m/s)
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05
Vr (m/s)
Vz (m/s)
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R (mm)
(g) p=0.12 MPa axial velocity distribution
20
5
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(f) p=0.08 MPa resultant velocity distribution
V (m/s)
0.030 0.028 0.026 0.024 0.022 0.020 0.018 0.016 0.014 5
15
R (mm)
(e) p=0.08 MPa radial velocity distribution
Vr (m/s)
Vz (m/s)
15
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05
R (mm)
(d) p=0.08 MPa axial velocity distribution
10
10
(c) p=0.04 MPa resultant velocity distribution
V (m/s)
0.024 0.022 0.020 0.018 0.016 0.014 0.012 0.010 0.008 0.006
R (mm)
0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 5
20
R (mm)
(b) p=0.04 MPa radial velocity distribution
Vr (m/s)
Vz (m/s)
(a) p=0.04 MPa axial velocity distribution
5
15 R (mm)
R (mm)
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05
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0.008 0.006 5
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(h) p=0.12 MPa radial velocity distribution
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R (mm)
(i) p=0.12 MPa resultant velocity distribution
Fig. 9. Test results by LDV.
Z= 0.31 face Z=0.28 face Z=0.25 face Z=0.22 face Z= 0.19 face Y
Z X
eter and the influence of boundary effect, the error of the measured results near the column axis and column wall is too large, and without regularity. Therefore, it is not indicated on the test result. It can be explained as that the test and the analog axial velocity distributions are basically the same. According to the comparison between Fig. 8e. Fig. 11b, we can find that: the radial velocity range of test result is 0–0.02 m/s, the radial velocity range of analog result is 0–0.05 m/s; from the center point to the position with distance of 0.013 m to the center Table 2 Boundary conditions and parameters for simulation.
Fig. 10. Lattice division.
Item Structure parameter
According to the comparison between Fig. 8d. Fig. 11a, we can find that: the axial velocity range of test result is 0–0.45 m/s, and the axial velocity range of analog result is 0–0.35 m/s; from the position with distance of 0.015 m to the axle center to the position with distance of 0.020 m to the axle center, the test and the analog axial velocity present the tendency of gradually increasing to the maximum; from the position with distance of 0.20 m to the column side wall, the analog axial velocity decreases rapidly to 0 from the maximum. Because of the limitation of Doppler Laser Velocim-
Water-phase property parameter
Parameter Height (m) Diameter (mm) Feeding rate (m/s) Discharging rate (m/s) Circulating pressure (MPa) Tangential feeding rate (m/s)
2 50 0.006539 0.11465 0.08
Density (kg/m3) Dynamic viscosity (Pa s)
998.23 1.005 103
0.796178
The velocity distribution rules of the cyclone section are shown in Fig. 11.
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4.00e-01
2.00e-01 1.00e-01
Radial velocity (m/s)
Axial velocity (m/s)
2.00e-01
Line 0.19 Line 0.22 Line 0.25 Line 0.28 Line 0.31
3.00e-01
0 -1.00e-01 -2.00e-01 -3.00e-01 -0.025
-0.015
-0.005 0.005 Position (m)
0.015
1.50e-01 1.00e-01 5.00e-02 0 -5.00e-02 -1.00e-01 -0.025
0.025
Tangential velocity (m/s)
2.00e-01 1.00e-01 0 -1.00e-01 -2.00e-01 -3.00e-01 -4.00e-01 -0.025
-0.015
-0.005 0.005 Position (m)
0.015
0.025
(c) Tangential velocity distribution of all sections
-0.005 0.005 Position (m)
0.015
0.025
(b) Radial velocity distribution of all sections
Relative velocity magnitude (m/s)
(a) Axial velocity distribution of all sections
-0.015
5.00e-01 4.50e-01 4.00e-01 3.50e-01 3.00e-01 2.50e-01 2.00e-01 1.50e-01 1.00e-01 5.00e-02 0 -0.025
-0.015
-0.005 0.005 Position (m)
0.015
0.025
(d) Relative resultant velocity distribution of all sections
Fig. 11. Velocity distribution rules of the cyclone section.
point, the analog axial velocity gradually increases from 0 to the maximum; from the position with distance of 0.013 m to the center point to the position with distance of about 0.020 m to the center point, both of the test and analog axial velocity present the tendency of gradual decrease; from the position with distance of 0.20 m to the center point to the column side wall, the analog axial velocity continuously and gradually decreases to 0. Due to the limitation of doppler laser velocimeter and the influence of boundary effect, the error of the measured results near the column axis and column wall is too large and without regularity. Therefore, it is not indicated on the test result. It can be explained as that the test and the analog axial velocity distributions are basically the same.
wastewater discharged at the overflow weir will cause oily concentration being lower, the quantity of wastewater for further treating will be increased, and so as the treatment cost. Therefore, a less volume of this part is better. The test determines the best operation condition: the feeding rate is 1.50 m3/h, the aeration rate is 2.50 m3/h, the circulating pressure is 0.28 MPa, and the underflow split ratio is 95%. (2) The relation between the cyclone separation efficiency R and the feeding rate gl, the aeration rate qg, the circulating pressure p, the underflow split ratio f is described as the following, which is also called the cyclone separation model:
R ¼ 6:2356 0:0385p0:05413 þ 0:0295qg
5. Conclusions
þ (1) Increasing the feeding rate, the oil removal will gradually decrease, the appropriate aeration rate is important for improving the cyclonic separation efficiency. With the increasing aeration rate to a certain value, the bubbles coalescence and fluid turbulence occur violently. The bubbles with smaller diameter, which are more capable for capturing fine oil droplets, gradually decrease and result in the oil removal decreasing. The circulating pressure is required to be set within an appropriate range. If the circulating pressure is low the centrifugal intensity will not be enough and the cyclone separation effect will be poor. While the pressure is too high, the oil droplets with larger diameter will be easily cut into small droplets, and the higher turbulence intensity of the cyclone separation section will cause low cyclone separation efficiency. Meeting the condition that the oil concentration of cleaning water discharged at the bottom outlet is a regulated content, and the larger of underflow split ratio is better. A bigger volume of oily
0:02161 þ 3:6250q1:4617 l
1:1855
4:4356f 0:3986
(3) Adopting the laser velocimeter, the axial velocity distribution rule of cyclone separation section was measured: within the range between the positions with distances of 0.005 and 0.02 m to the center point, the velocity increases with the increasing distance to the center point, the measuring range of axial velocity is 0–0.7 m/s; the radial velocity distribution rule: within the range between positions with distances of 0.005 and 0.02 m to the center point, the velocity decreases with the increasing distance to the center point, the measuring range of radial velocity is 0–0.03 m/s. (4) The axial velocity distribution rule of cyclone separation section is acquired by the cyclone numerical simulation results: the axial velocity presents the tendency of decline when going upward or downward along the tangent feeding surface Z = 0.25; generally speaking, the axial velocity are mostly negative value, and the velocity direction is downward along the axis; the radial velocity distribution
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rule: the radial velocity is less than the axial velocity; from the column wall to the axis center, the radial velocity distribution rule is that rapidly increasing at first and then decreasing to 0; the radial velocity direction is from the column wall to the axle center; the tangential velocity distribution rule: the tangential velocity is larger than the axial velocity and the radial velocity; it basically presents as the symmetrical distribution; the value of negative half axis at the side of tangential inlet is larger than the value of positive half axis at the other side; from the column wall to the center point, the tangent velocity rapidly increases to the maximum at first, and then slowly decreases to 0 at the axis center. According to the comparative analysis conducted to the test and analog results by applying the two velocity components of axial velocity and radial velocity, we know that the regularities of the analog and test results are basically the same, which verifies the reliability of the numerical simulation. Acknowledgment The authors are grateful to the National Natural Science Foundation of China (No. 50974119) for the financial support for this project. References [1] Emamjomeh MM, Sivakumar M. Review of pollutants removed by electrocoagulation and electrocoagulation/flotation processes. J Environ Manage 2009;90(5):1663–79. [2] Li XB, Liu JT, Wang YT, Wang CY, Zhou XH. Separation of oil from wastewater by column flotation. J China Univ Min Technol 2007;17(4):546–51. 577. [3] Capponi F, Sartori M, Souza ML, Rubio J. Modified column flotation of adsorbing iron hydroxide colloidal precipitates. Int J Miner Process 2006;79(3):167–73.
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