Separating oil from oil-water emulsions by electroflotation technique

ELSEVIER

Separations Technology 6 (1996) 9-17

Separating oil from oil-water emulsions by electroflotation technique Ashraf Y. Hosny Materials Science Depamnent, Institute of Graduate Studies and Research, Alexandria Uniuersily, Alexandria, Egypt

Received July 10.1995; accepted November 20, 1995

Abstract The separation of finely dispersed oil from oil-water emulsions was carried out in an electroflotation cell which has a set of electrodes, a lead anode and stainless steel screen cathode. The effect of operating parameters on the performance of the batch cell was examined. The parameters investigated are electrical current, oil concentration, flotation time and flocculant agent concentrations. A well-fitted empirical correlation represents the change in percentage oil removal with wide range of operating conditions was obtained. The oil separation reached 65% at optimum conditions; 75% in the presence of NaCl (3.5% by wt. of solution); and 92% with the presence of NaCl and at optimum concentration of flocculant agent. Electrical energy consumption varied from 0.5 to 10.6 KWh/m3 according to experimental conditions. An equation relates the K with I was obtained. The general form of the equation is K = constant (I)“; where the n values are 0.64 and 0.62 for solutions with and without NaCI, respectively. The previous relation is valid only for current values from 0.3 to 1.2 A. The effect of emulsion flow rate on the separation process was determined on continuous scale. Keywords:

Oil separation;

Flotation

technique;

Electroflotation;

1. Introduction

The field of separation of emulsions or colloidal particles from water is a major concern for several industries such as petroleum, food, pulp and paper. The need to have an efficient and quick method for separation motivates these industries to develop alternative means than traditional processes. Electroflotation techniques are highly versatile and competitive to settling tank techniques which requires large land space. It is also competitive to other flotation techniques such as dissolved air flotation and dispersed air flotation. The electroflotation units are small and compact and require less maintenance and running cost than other flotation units [1,23. The electroflotation technique depends upon generation of hydrogen and oxygen gases during electrolysis of water. Gas bubbles formed on electrode surface contact with oil

Waste oil removal

drops; then the attached oil-gas combinations rise up to the surface where oil may be removed by any skimming method. Waste petroleum industries usually have large volumes of small oil concentrations in their effluents, which form stable oil-water emulsions [3]. Oil effluents can result also from the remaining of oil spill disasters in sea waters. Currently several water desalination plants face problems in filters’ units due to presence of oil emulsions in their intake water sources. Electroflotation technique has three principal advantages. First, dispersed gas bubbles formed from electrolysis are extremely fine and uniform, (with average bubble diameter around 20 ,um>. Second, varying current density gives the possibility of varying any gas bubble concentrations ‘in the flotation medium, thereby increasing the probabilities of bubble-oil drop collision. Third, selection of appropriate electrode

0956-9618/96/$15.00 0 1996 Elsevier Science Ireland Ltd. All rights reserved. SSDI 0956-9618(95)00136-T

10

A. Y. Hosny / Separation Technology6 (I 996) 9-17

surface and solution conditions permit one to obtain optimum results for a specified separation process [3-51. A literature survey revealed a number of studies that demonstrate successful oil separation from oilwater emulsions using the electroflotation technique [6-121. Most of these studies utilized soluble anodes such as Fe or Al coupled with Pt cathodes and flocculating agents to improve the flotation process. A few studies applied insoluble anode for oil removal processes [11,12]. The need to lower the cost of electrode materials and control the current densities are motives to investigate in depth the performance of an electroflotation ccl1 applied to oil separation from waste effluents. The present study aimed at investigating the effect of operating conditions such as electrical current, initial oil concentration and flotation time upon the performance of electroflotation cell equipped with insoluble electrodes. The electrode’s materials were lead as anode and stainless steel screen as a cathode. The study also evaluated the synergistic effect of NaCl and flocculant agent additions to the emulsion on the performance of the entire process. The experimental cell was a modified version of the one used in our previous work and was allowed to determine the oil concentration inside the column [12]. A correlation is given to correlate the various operating parameters in order to asset designers, operators and investigators. The optimum conditions are tested on the continuous scale. 2. Parameters

affecting electroflotation

process

The rate of the flotation process is affected greatly by the size of floating particles [3]. Fukui and Yuu [17] studied the collection of micron particles in electroflotation. Their results showed that the rate of flotation strongly depends upon the charge of both particle and bubble. However, the measurement of the charge on a small gas bubble is not easy to determine and only a few investigators measured bubble zeta potentials [18]. The results indicate that maximum rate of flotation was achieved when the zeta potentials of bubbles and particles were in opposite sign. The flotation process is also affected by the bubble size of the hydrogen and oxygen gases formed at electrode surfaces. There are several factors influence bubble size such as current density, temperature and curvature of electrode surface, but the greatest effect occurs from electrode material and pH of the medium. Moreover, the motion of the bubble inside the cell is important to the flotation process. The trajectory of the bubbles affected by the hydrodynamics and electrodes position in the cell. The success of flotation process depends upon the presence of sufficient volume of gas bubbles relative to the floating parti-

cles. However, large volume of gas bubbles could lead to coalescence of bubbles instead of attachment to floating particles. This criterion is difficult to adjust in conventional flotation processes, however, it is relatively easier to adjust in an electroflotation process. Mathematical models have been developed to describe the kinetics of the process. Nazaryan [8] developed a theoretical model of the kinetics of the coagulation of waste water impurities by electrochemically produced coagulant. Tyabin [191 derived a model based upon experimental data obtained from industrial effluents of food products. Hosny [12] correlated the separation of oil from emulsions by a first order kinetic rate model. 3. Experimental

details

The flotation cell was a plexiglass in rectangular shape (8 cm x 9 cm) and 30 cm in height. A sampling valve fixed 1 cm above the cell bottom, several valves were located also at several locations from the cell bottom (7, 13 and 19 cm). The emulsion volume in the batch cell was 1.5 1 and increased to 2.5 1 when the experiments were carried out to investigate the distribution of oil concentration in the cell at several locations. The cathode was a stainless steel screen (wire diameter of 0. 16 mm and wire intersections of 400 per cm*), and positioned horizontally at the top of a lead anode. The gape between the two electrodes was 1.5 cm. The anode fixed at 1.5 cm above the cell bottom. The area of the cathode was equal to 120 cm* calculated according to equation by Alexeyev [13] and current density calculated according to this area. The electrical circuit consisted of a DC power supply, an ammeter connected in series and voltammeter in parallel. The temperature was kept constant at 25 + 1°C. To simulate the dispersion of oil in waste water, Nalco 7313 emulsifier was used to prepare an oil-water emulsion. The emulsifier concentration used was 350 mg/l corresponding to CMC value in order to produce a stable emulsion. The CMC value determined in previous work [12]. The oil used in the experiments was Marine oil, and the analysis given in Table 1. The oil-water emulsion prepared by vigorous mixing of emulsifier with the desired oil concentrations (0.5-2.0 g/l) as method described in [12-141. The oil size of stable emulsion varied from l-5 pm; the size distribution of oil drops in the emulsion is as follows: 1 firn or less = 38%; 2 pm = 25%; 3 pm = 20%; 4 pm = 10% and 5 pm = 7%. The pH of the emulsion adjusted at 4.5 according to previous. work [123. The flocculant agent was cationic emulsifier Nalco 7720 from NALCO company. The flocculant agent concentrations varied from 4 to 32 mg/l. The concentrations of NaCl add to solutions was 3.5% by wt. to simulate the concentra-

11

A. Y Hosny / Separalions Technolom 6 (19961 9-I 7 Table I Properties for oil-water

of Marine

Balaiem

crude

oil used as starting

material

emulsion

Property

Value

Specific gravity at 288.6 K Viscosity Redwood no I at 310.8 K SEC Flash point PM closed K Sulphur content % wt. Wax content % wt. Asphaltene % wt.

0.9465 1350 167 2.66 3.57 3.93

tion of salt in sea water. Fig. 1 shows the schematic diagram for the continuous process. The flow rates varied from 10 to 50 ml/min. The oil samples were determined till steady state reached. The oil concentrations determined by extraction-calorimetric technique [7,12]. 4. Results and discussion 4.1. Batch cell 4.1.1. Current

Fig. 2 shows that percentage oil removal increases with the increase in current values up to 1.2 A (optimum value); where a maximum of 65.4% oil removal was obtained. A further increase in current up to 3.6 A reduce oil removal to 59%. Initially, increasing current enhances the generation of hydrogen and oxygen gases formed at electrode surfaces. This leads to an increase in the number of gas bubbles inside the cell. Consequently, the attachment step between gas bubbles and oil drops is enhanced, and more oil drops are carried out by gas bubbles. However, further increase in current beyond the optimum value increases excessively the number of bubbles generated.

Thus there is a strong likelihood for bubbles to coalesce together instead of attachment with oil drops [3,12]. The results in Fig. 2 also indicate that there is a significant enhancement in oil removal due to the presence of NaCl. The oil removal increases from 65.4 to 74% at current equal to 1.2 A. Previous studies 13,151 showed that NaCl presence decreases the size of gas bubbles, especially hydrogen gas. Since the buoyancy of smaller bubbles is lower than larger bubbles, they rise slowly to the surface with high opportunities for collision with oil drops. This leads to an improvement in the oil removal process. Fig. 3 indicates that electrical energy consumption increases with increasing electrical current applied. Since electrical current is a key variable in controlling the performance of the flotation process, it is desirable to decrease cell voltage rather than decreasing current in order to minimize energy consumption (Ohm’s law>. The conductivity of the emulsion affects greatly the cell voltage (Table 2). Fig. 3 shows the effect of NaCl in decreasing electrical energy consumption. For example, at 1.2 A current and after 40 min flotation time, the electrical energies are 4.25 and 2.66 kWh/m” for emulsions without and with NaCl additions, respectively. This is almost 62% reduction and in agreement with those reported in literature [6]. The results suggest that oil removal will be less expensive in waste oil emulsions originated from sea waters. The rate of oil removal oil from oil-water emulsions can be expressed by the following equation: - VdC/dt

= KAC

(1)

For the isothermal

case, the preceding equation can

water 011

1"

1 IIII

Emulsrfier

I

3ver

-

Ilow

-1” Flow meter 0

-*

ii4

?! 3

ii?

electrodes

1utlet # I

-I-Feedmg Tank

Cell Column

Feedmg Pump

Fig. 1. Schematic

diagram

for continuous

electrotlotation

cell.

A. Y. Hosny /Separations

12

Technology 6 (1996) 9-I 7

85

12 1

1

0

2

Current

3

0.3

4

0.6

0.9

1.2

Current

(amp.)

1.5

1.8

2.1

2.4

(amp.)

Fig. 2. The effect of current on percentage oil removal, flotation time 40 min.; oil concentration 1050 me/l.

Fig. 3. Electrical energy vs. current, flotation time 40 min.; oil concentration 1050 mg/l.

be integrated

increasing current. While in the second stage, K decreases with increasing current. The second stage is presented here to emphasize on the concept of optimum current value. From the point of oil removal, the first stage is important (up to optimum current value). Therefore, equations relate (K) to (I) are driven from the plot for the first stage and as follows:

to give the following:

V In C,/C, = ZL4t

(2)

To show that Eq. (2) can fit the experimental data obtained, a plot between (In C,/Cf) versus (t) is drawn. Straight lines were obtained as shown in Fig. 4 (a-d) and the K values were calculated from the slope of the lines. Figs. 4a and 4b represent emulsions without and with NaCl, respectively, for current values up to the optimum value (first stage). While Figs. 9c and 9d are the presentation beyond the optimum current value (second stage). Table 2 lists the K values obtained with different operating conditions. The higher K value means a higher percentage of oil removal. Fig. 5 shows the relationship between log K and log Z and the relationship is divided into two stages. The first stage indicates that K increases with

K- u,Z’.~~ with NaCl and Z from 0.3 to 1.2 A

(3)

K= u~Z’.~* without NaCl and Z from 0.3 to 1.2 A (4) Where a, = 5.71 X lop3 and a2 = 4.23 X 10e4. The exponents in the equations are very close which means that the addition of NaCl does not change the mechanism of the system. In case of using current density (25-100 A/m*> instead of current, the exponents remain the same and the constants are = 3.36

Table 2 The change of both cell voltage and removal rate constants with current values; conditions are: flotation time 40 min., initial oil concentration 1050 mg/l, and cell volume 1.5 I Current (A)

0.3 0.5 0.8 1.2 2.4 3.6

Current density

25 42 67 100 200 300

Cell removal rate constant (cm/s) X lo3

Cell voltage (Volt) Without NaCl

With NaCl

Without NaCl

With NaCl

4.0 5.4 7.2 8.0 10.0 13.5

2.2 3.0 4.3 5.0 8.6 12.5

2.08 2.66 3.54 5.01 4.65 4.26

2.68 3.32 5.36 6.34 6.03 5.40

Trch,7olog?

(a)

1 .6

‘c)

,

0 11996)

1.6 Current (amp )

Current (amp ) 708

+12

without

NaCl

10

*05

9-I 7

H3.6

HO3

( first stage)

20

30

without

40

+24 NaCl

10

50

20

30

40

50

40

50

Cd) 2

2

Current (amp.) f-l 1.6

( second stage)

Time (min)

Time (min) (h)

t-12

-

2 +tO 8 WO.5 with

NaCl

10

Current (amp ) *Co.3

H36

+24

A-l.2

(first stage)

20

30

40

50

Time (min)

10

20

30

Time (min)

Fig. 4. (a) In C,,/ CI vs. time for emulsion without NaCl addition, oil cont. 1050 mg/l (first stage). (b) In C,,/ CI vs. time for emulsmn with NaCl addition. oil cont. 1050 mg/l (first stage). (c) In C,,/ Cf vs. time for emulsion without NaCl addition. oil cont. 1050 mp/l (second stage). (d) In C,,/ c‘, vs. time for emulsion with NaCl addition, oil cont. 1050 mg/l (second stage).

x 10.’ and a, = 2.74 x lo-‘. Eqs. 3 and 4 developed can be used to predict the K values and can be a base for comparative purposes.

4.1.2. Flotation time Fig. 6 shows the variations in oil concentrations with time at initial oil concentration equal to 1050 mg/l. For flotation times of 40 and 60 min, the oil concentrations are 354 and 265 mg/l, respectively. The oil removal values are 65.4% and 71 %, for 40

and 60 min, respectively. The corresponding electrical energy increases by a factor of 1.5 while oil removal enhances by a factor that does not exceed 1.1. The trend of the present results is similar to our previous work [12]. From the point of energy efficiency, 40 min may be considered an optimum flotation time. 4.1.3. Oil concentration The results in Fig. 7 show that increasing oil concentrations (500-2000 mg/l) enhances the percentage

A. Y. Hosny /Separations

14

Technology 6 (1996) 9-I 7

-1.75 I without NaCl

()with

NaCl

??

0

80 -2

70 60 50 40 30 20

-2.75

10

1~ -3_

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

Fig. 5. Effect of current on the K constant.

1200

1000

800

g 600

3 r= 0

400

2oa

C

0

10

20

30

40

50

60

Flotation Time (min.) Fig. 6. The change of percentage current = 1.2 A.

20

30

Fig. 7. The effect of oil concentrations current = 1.2 A.

oil removal. For example, the percentage oil removals, after 40 min, are almost 55, 65 and 70 for initial oil concentrations 490, 1050 and 1990 mg/l, respectively. The enhancement in oil removal may be due to an increase in the chance of gas bubbles to attach to floating oil drops in the emulsion. The results show that for all the initial oil concentrations; the percentage removal starts to stabilize after specific time (40 min). The oil drops inside the emulsion

2

10

oil concn. (mgA) *1000 81500 A2000 40

50

60

Flotation Time (min.)

Log I

s

*500

oil concentration

with time,

on percentage oil removal,

have several sizes, once the largest drops are removed, the efficiency of the process slows down. Literature review indicated that smaller oil drops can not be removed from waste emulsions by electroflotation unless their size is increased [8]. This leads to an enhancement in collision probability between oil drop and gas bubble. 4.1.4. Flocculant agent The change in percentage oil removal with addition of flocculant is shown in Fig. 8. The oil removal increases from 65% to 82 % as the flocculant agent is added with a concentration equal to 32 mg/l. The flocculant agent improves the percentage oil removal by aggregating smaller oil drops into larger entities suitable for contact with gas bubbles. The percentage oil removals after 40 min are 65, 73, 79, and 82 for 0.0, 8, 16 and 32 mg/l, respectively. It is worth to notice that only a 3% increase in oil removal is obtained when the concentration of flocculant agent is doubled from 16 to 32 mg/l. Generally, the optimum concentration for flocculant agent must be determined to decrease the cost of the process. Once the threshold of flocculant is reached, further increase in concentration does not affect the separation of the process. The combined effect between NaCl and flocculant agent is shown in Fig. 9, where the percentage oil removal enhanced greatly to a value equal to 92%. 4.1.5. Experimental correlation An empirical equation is introduced

to correlate the entire data obtained under several operating con-

A. Y Hosny /Separations

15

Technology 6 (199619- 17

90 80 70 8

60

‘;; $

50

E

40

2 5

3o 20 flocculant concn. (mgil) 84 t8 t12 ~16 832

10 ’ +O 0 0

10

5

15

20

25

30

35

20

40

Flotation Time (min.)

Flotation Time (min.) Fig. 8. The effect of flocculant agent concentrations on the percentage oil removal, current = 1.2 A and oil concentration 1050 mg/l.

ditions. The equation developed sion analysis is as follows:

using linear regres-

Percentage oil removal = constant +9.18x

1O-3 (Cl +1.09(t)

+27.13 (I) (5)

The equation is valid only for (0.3-1.2 A), t (lo-40 min) and C (500-2000 mg/l). The constant in the correlation is = -22.3. The equation developed fits the experimental data very well as shown in Fig. 10 with average deviation of +2.0% and the regression coefficients are R = 0.994 and R* = 0.988. The correlation is useful for comparative results with other studies and assists evaluation of wide range of operating parameters. 4.1.6. Cell height The oil concentrations inside a flotation cell (2.5 1) change according to the height from the bottom of

Fig. 9. The effect of flocculant agent and NaCl on the percentage oil removal.

the cell. The concentration increases with increasing the distance from the bottom of the cell. Table 3 lists the oil concentration values at several positions (l-19 cm). Table 4 shows the change of the cell removal rate constant, K, as a measure of cell performance for several positions and emulsion conditions. The values of cell constant decreases as the height increases. Where, the oil tend to accumulate at the surface and clear water at the bottom of the cell. The oil concentration values shown at position (1 cm> are 507 and 495 mg/l for NaCl case and flocculant agent, respectively. The values obtained are almost equal, however, this is not the case at higher locations. This suggests that the hydrodynamics inside the cell has an influence on the process. 4.2. Continuous process The effect of emulsion flow rate on the oil concentration in the effluent is shown in Fig. 11. Increasing emulsion flow rate decreases percentage oil removal,

Table 3 The distribution of oil concentrations inside the cell at several positions, conditions are; flotation mg/l, current 1.2 A and cell volume 2.5 I Effluent oil concentrations Positions

Emulsion conditions

No additives NaCl 3.5% by wt. Flocculant (16 mg/l) Flocculant (16 mg/l)

plus NaCl 3.5% by wt.

30

time 40 min.; initial oil concentration

(mg/l)

1 cm

7 cm

13cm

l9cm

590 507 495 229

676 566 549 314

725 608 620 410

745 615 690 463

10X1

A.Y. Hosny /Separations Technology6 (1996)9-17

100

C m%A).I ( A) 0

1000,

0.3

A

1000,

0.5

*with

NaCl

plus flocculant

agent

?? lOOO, 0.8 x 1000, 1.2

I ISOO. 1.2 0 1500. 1.2 12000,

';;j

1.2

'

E 2 r=

60'

0

40

10

0

20

30

40

50

60

70

20

80

Calculated Oil Removal (%) Fig. IO. Experimental

measurements

lated values from correlation;

flotation

for oil removal

30

20

10

50

40

Flow Rate (ml/min.)

versus calcu-

times are 10, 20, 30,40

Fig. 11. Effect of emulsion flow rate on percentage

oil removal.

min.

e.g., the percentage oil removals are 63,56, and 36 for flow rates 10, 30 and 50 ml/min., respectively. The value of oil removal in continuous process at low flow rate is close to the values obtained from the batch process. Increasing flow rates means decreasing residence time available for oil drops to contact with gas bubbles. Higher flow rates is a necessity for industrial applications, in order to treat higher volumes of emulsified oil solutions. A balance between higher flow rates and separation efficiency must be taken in consideration on practice. Flocculant agents also improve the separation process on continuous scale as well as batch scale. For example, at 10 ml/min emulsion flow rate, the percentage oil removals are 63 and 89 for emulsions without additives and with additives, respectively. Flocculant agent 16 mg/l is used in the continuous process. An attempt was made to fit the behavior of the cell to either ideal plug flow or mixed flow reactor. The calculations indicate that the mixed

flow reactor is more suitable to represent the performance of the cell. However, a more complicated model must be attempted in future. These models compensate the non-ideality in the pattern of the flow. 5. Conclusions The oil removal increases with electrical current up to a certain value (1.2 A) equivalent to 100 A/m2, then oil removal declines. The cell performance improves with increasing oil concentration and flotation time. The maximum oil separation is 92% obtained at optimum conditions such as 1.2 A, 40 min and 3.5% by wt. NaCl and 16 mg/l flocculant agent. The increase in K values means an enhancement in the oil removal. Exponential relations are obtained between K and I, the exponents are 0.64 and 0.62 for emulsions with and without NaCl addition, for Z(O.3-1.2

Table 4 The cell removal current

constants

at several positions inside the cell;

conditions

are: flotation

time 40 min.; initial oil concentration

1050 mg/l,

1.2 A and cell volume 2.5 I Cell removal rate constant (cm/s)

Emulsion conditions

X IO3

Positions I cm

7cm

I3 cm

l9cm

No additives

5.01

3.80

3.21

2.97

NaCl3.5%

6.34

5.36

4.74

4.64

5.62

4.57

3.64

10.47

8.15

7.09

by wt.

Flocculant

(16 mg/l)

Flocculant

(I 6 mg/l)

6.52 plus NaCl 3.5% by wt.

13.2

A). The oil concentration

increases from the bottom to the top of the cell. Experimental correlation was developed to correlate the operating parameters with the percentage oil removal. The correlation fits the experimental data very well. In continuous processes increasing emulsion flow rate decreases the rate of oil removal. Presence of flocculant agent and NaCl assist the separation process on continuous and batch scale. Electrical energy consumption is a crucial factor in evaluating the potential application of electroflotation techniquse using insoluble anodes. The energy consumption in electroflotation technique decreased significantly in presence of NaCl. This makes electroflotation technique a useful method in removing oil emulsions formed after accidents of oil spills in sea waters. The concept of application of insoluble electrodes in electroflotation is an attractive concept that can be applied in oil separation as well as other separation processes. Notation

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