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Interfacial studies on dissolved gas flotation of oil droplets for water purification
Colloids and Surfaces A: Physicochemical and Engineering Aspects 154 (1999) 127 – 135
Interfacial studies on dissolved gas flotation of oil droplets for water purification R.C.G. Oliveira a, G. Gonzalez a, J.F. Oliveira b,* a
Petrobras Research Center (CENPES), Cidade Uni6ersita´ria, Quadra 7, ZIP: 21949 -900, Rio de Janeiro, RJ Brazil b Department of Materials and Metallurgical Engineering, COPPE/UFRJ, Federal Uni6ersity of Rio de Janeiro, Cidade Uni6ersita´ria, Caixa Postal 68505, Rio de Janeiro, RJ Brazil
Abstract The removal of emulsified residual oil from water is an important issue due to the increasing concern with the environment. In the present paper various surface chemistry aspects of dissolved gas flotation were studied aiming at a better understanding of the mechanisms of the process. These studies included surface and interfacial tension measurements of a model system composed of n-dodecane in the presence and absence of nonyl-phenol. Critical micelle concentration and spreading coefficients determinations were undertaken at different NaCl concentrations. The spreading coefficient proved to be a critical parameter in oil droplets flotation. After bubble – drop contact and rupture of the water film, oil drops have a tendency to spread over the bubble surfaces. The spreading coefficient increased with increasing salt concentration. An experimental setup was designed to measure the induction contact time between single n-dodecane droplets and air bubbles. Induction time decreased with increasing salt concentration. In the presence of the nonyl-phenol aqueous solution, induction time decreased with time of stabilization. On the other hand, induction time increased with time of stabilization when nonyl-phenol was added to the organic phase. The mechanisms involved are discussed and some results obtained in a dissolved air flotation batch unit in which bubbles size was measured and controlled are also presented. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Dissolved gas flotation; Bubble–drop attachment; Flotation efficiency; Oily water cleaning
1. Introduction The effect of particle size in the flotation process has been the subject of research work in mineral processing by several authors. An excellent review has been presented by Trahar [1] in which peculiarities related to the flotation of fine, intermediate and coarse sized particles have been * Corresponding author. E-mail address: [email protected] (J.F. Oliveira)
approached independently. As far as the efficiency of collision is concerned, bubble–particle and bubble–drop contact phenomena are very similar. For this reason, most of the studies in mineral processing are also relevant to the petroleum industry. The efficiency of fine particles flotation has been the subject of some theoretical and experimental studies as indicated by reviews presented by Trahar and Warren [2], Jameson et al. [3], King [4], Linch et al. [5] and Shulze [6]. More recently, Deglon and O’Connor [7] investigated
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the floatability of fine particles in a conventional flotation cell and confirmed the strong dependence of the flotation rate constant on the particle and bubble sizes. Their results also indicated a maximum in the flotation rate constant as a function of these factors and of the pulp agitation.
1.1. Mechanisms of collision and attachment The recovery in flotation starts with the adhesion of particles to the air bubbles followed by transportation of the aggregate to the froth, drainage and enrichment of the froth and finally by its overflow removal from the cell top. However, each of these process phases occur, in fact, in several stages. The formation of a stable bubble – particle or bubble–drop aggregate is generally considered to be the rate controlling step in flotation. The first necessary condition for its effectiveness is that the mutual trajectories lead to a collision stage. Once the bubble is generally much larger than the particle, the situation is similar to that of a particle approaching a plane surface. Moreover, because of the elasticity of the bubble a depression is formed during the collision stage. Philippoff [8] and Evans [9] developed a theory to calculate the contact time based on the hypothesis that the bubble absorbs the kinetic energy of the particle during the collision. However, for flotation effectiveness it is also necessary that during this contact or induction time the thinning and rupture of the liquid film between the bubble and the drop or solid particle also occur. Derjaguin and Dukhin [10] have introduced the concept of disjoining pressure which can be applied to the bubble – particle interactions. According to this theory, when the liquid film reaches a thickness close to 0.1 mm, very strong molecular forces come into effect which could lead to the rupture of the film and the effective bubble–particle attachment. However, the shining and rupture of the film need to be followed by the expansion of the contact meniscus formed at the point of rupture which will give rise to the establishment of a contact angle large enough between the particle
and the bubble. All these stages must take place before the particle is bounced off the bubble surface when it recovers its original spherical shape.
1.2. The influence of particle and bubble sizes in flotation The attachment mechanisms briefly discussed led some authors [10–14] to formulate the flotation rate problem as a probability of flotation (Pf) according to the expression: Pf = Pc·Pa·Ps
(1)
where Pc is the probability of collision; Pa, the probability of adhesion (thinning and rupture of the liquid films during contact time) and Ps, the probability of a stable aggregate formation (a function of contact angle value). The probability of bubble–particle collision (Pc) has been directly related to physical variables such as bubble diameter, density of particles, viscosity of the pulp, relative velocity between the particle and bubble and, particularly, to the particle size. In the case of oil droplets, their low density certainly plays an important role in the collision with ascending bubbles generated by dissolved gas flotation. Reay and Ratcliff [11] studied the flotation of fine particles by dissolved gas flotation and proposed hydrodynamic equations for the movement of the particles in relation to the bubbles. They have found that the collision efficiency (Ec) is related to the diameter of the particles (dp) and that of the bubble (db) by the expression: Ec = a0
dp db
N
(2)
where Ec is the relation between the number of particles that effectively collide with a bubble and the number of particles laying along its cylindrical trajectory; a0, the proportionality factor; N, an exponent depending on the density of the particles (rp) and that of the fluid (rf); (N= 1.90 for rp/rf = 1.0 and N=2.05 for rp/rf = 2.5). Previous theoretical studies by Flint and Howarth [12] have led to a similar equation:
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129
Fig. 1. Schematic diagram of the dissolved gas flotation batch unit: (1) Pressure chamber; (2) Bubble generation system; (3) Flotation cell.
Ec =a1
dp db
2
(3)
where a1 is also a proportionality constant. In this last expression, the volume of each particle was disregarded and the limiting condition for collision is considered to occur when the center of the particle touches the surface of the bubble. The Reay and Ratcliff [11] hypothesis for
Fig. 3. Spreading coefficient as a function of salinity for different concentrations of nonyl-phenol.
the limiting condition, however, is that in which the particle surface just touches the bubble surface. For particles smaller than 0.2 mm, Reay and Ratcliff [11] deduced the following expression for the particle–bubble collision efficiency: Ec =
n
1.5 0.5 R2 1+ G− + R R3 (1+G)
(4)
Where R and G are given by: R= 1+
dp db
G= (R 2 − 1)·
Fig. 2. Surface tension of aqueous solutions as a function of nonyl-phenol concentration for the salinities specified.
n rp −1 rf
(5) (6)
Eqs. (3) and (4) show that for small particles the efficiency of collision increases as particle size increases and bubble size decreases. The hydrodynamic theory is based on the assumption that electrical interactions between particle and bubble are not significant. However, the theory of Derjaguin and Dukhin [10] for the rupture of the liquid film leads to the conclusion that a very strong electrical field may sometimes originate locally.
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Studies by Collins and Jameson [15] with polystyrene particles as well as those by Reay and Ratcliff [16] with glass beads showed an Ec = d 1.5 p for the case of small bubbles ( B 100 mm). Anfruns and Kitchener [17] found that Ec =d 2p for bubble sizes around 1 mm.
1.3. Beha6ior of oil drops after attachment The flotation of oil drops presents some peculiarities. First, it is important to consider that unlike solid particles the droplets are not rigid bodies and suffer deformation by the hydrodynamic effect. Their low density also has an influence in the collision stage. However, the most characteristic effect is the spreading of the oil on the bubble surface after the attachment takes place [18]. The tendency of the oil to spread as a film on the liquid – gas interface is dictated by the positive value of spreading coefficient (Sow) defined by the following expression [19]: Sow = gw −(go +gow)
(7)
where gw is the surface tension of pure water; go,
Fig. 4. Bubble–drop attachment process: (a) approach; (b) bubble – drop contact (induction time start); (c) bubble – drop attachment (induction time end); (d) interfacial film spreading.
the oil surface tension and gow, the interfacial tension between oil and water. For the case of n-dodecane, its surface tension is go = 26.1 mN m − 1 and the interfacial tension with water is gow = 41.6 mN m − 1. Thus, for gw = 71.4 mN m − 1, Sow = + 3.7 mN m − 1. This is a low value, but still leads to the spreading of the oil on the bubble surface. Fortunately, the spreading coefficient is positive for most of oil–water systems, enhancing the attachment to the bubble surface. Roques et al. [20] have studied the bubble–drop coalescence phenomena and found that the presence in the bubble of a gaseous compound transferable to the continuous aqueous phase favours the coalescence. In this way, the addition of NH3 to the air used in the flotation process considerably increased the coalescence thereby enhancing the kinetics of the process.
1.4. Inno6ations in flotation equipment An excellent review of the recent developments in flotation equipment has been presented by Brewis [21]. Concerning the flotation of oil droplets, the Jameson cell is an equipment that, due to its fast flotation kinetics and great compactness, has been used in several plant sites for the removal of organic components from hydrometallurgical liquors in SX–EW circuits [21]. This could lead to its application also in the cleaning of effluent water from the petroleum industry. However, the dispersed air flotation (DAF) has been the most widely used process for the flotation of oil drops from produced water. It consists basically of a saturator where the gas is dissolved under pressure. After introduction into the flotation cell, the contaminated water becomes supersaturated and releases the air as small bubbles. The process has been introduced for the removal of fine particles in the treatment of effluents [22]. Recently, with the worldwide environmental concern in relation to the petroleum industry, the DAF process has become widely used in the water purification stage. In the present paper, some aspects of the mechanisms of oil flotation and DAF were studied and are discussed.
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Fig. 5. Induction time as a function of the stabilization time for different nonyl-phenol concentrations added to the aqueous and organic phase.
2. Experimental
2.1. Materials and procedure The n-dodecane used was obtained from Merck (Sa˜o Paulo, Brazil) with a 99% purity. Pure nonyl-phenol, molecular weight= 225 and HLB =5.0, was supplied by Oxiteno (Rio de Janeiro, Brazil). Double distilled water, further purified by the milli-Q system, was used throughout the experiments. Its resistivity was 18.2 MV cm and surface tension around 71.5 mN m − 1 at 25°C. Other reagents used were of analytical grade. Surface tension measurements were conducted with a Kruss K10 tensiometer using the ring method. The size of the drops in the n-dodecane prepared emulsions was measured using a
Malvern 3600/E laser light scattering analyser. The determination of the initial and residual concentration of oil in water, to control the efficiency of flotation experiments, was conducted using an Horiba OCMA-220 infrared analyser.
2.2. Special apparatus To determine the bubble sizes a special rectangular cell was constructed to which the feed could be introduced via a bypass system and photographs could be taken. The optical system of a Rame´-Hart goniometer was used with a camera attachment. The determination of bubble–drop induction time needed for coalescence was carried out using a similar optical system to follow the vertical approach of a single pair of drop and bubble and
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Fig. 6. Induction time as a function of stabilization time and salinity for different NaCl concentrations with 10 − 5 M of nonyl-phenol added to the organic phase.
measure the contact time before coalescence [23]. The time of stabilization, defined as the time of immersion of both bubble and drop in the medium before contact, was also a parameter studied. The size of the bubble generated was equal to the external diameter of the capill-ary tube (3.2 mm) and that of the drop (1.4 mm). The DAF batch unit used in this work is composed of a pressure chamber, a bubble generation system and the flotation cell. The schematic diagram of the DAF batch unit is shown in Fig. 1 and details of the methods used can be found elsewhere [23].
3. Results and discussion Based on surface tension measurements, the critical micelle concentration (CMC) and spreading coefficient were determined for several experimental conditions. The induction or contact time necessary for attachment was measured as a function of the time of stabilization or immersion of drops and bubbles in the medium before contact.
3.1. Critical micelle concentration and spreading coefficient Results of surface tension measurements as a
R.C.G. Oli6eira et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 154 (1999) 127–135
function of nonyl-phenol concentration are presented in Fig. 2. It can be seen that although an increase in the NaCl concentration to 3 and 6% has some effect in the surface tension at low concentration of nonyl-phenol, the CMC value remains constant around 7× 10 − 5 M. Two values of surfactant concentration lower than the CMC were thus chosen to calculate the spreading coefficient as a function of salinity (Fig. 3). In aqueous medium, the hydrocarbon droplets presented a natural tendency to adhere to the bubbles surface. Nevertheless, for the attachment to take place, the spreading coefficient should, in general, present a positive value. Because of the presence of curved interfaces, a capillary pressure contribution can, in some situations, lead to the spreading of the oil even if the spreading coefficient presents a negative value. As far as the salinity is concerned, the spreading coefficient increased slightly with increasing NaCl concentration. The effect of nonyl-phenol, however, is very striking. Fig. 3 shows that for a surfactant concentration equal to 10 − 5 M the spreading coefficient decreases to slightly negative values depending on the salinity of the water
Fig. 7. Bubble diameter as a function of nonyl-phenol concentration and salinity.
133
medium. This means that the natural presence of surfactants in crude oil should be taken into consideration in the water purification by flotation, due to their decreasing effect on the spreading coefficient.
3.2. Influence of stabilization time on the induction time Fig. 4 illustrates the attachment process, which is characterized by the rupture of the aqueous film between the bubble and the drop, followed by the spreading of the oil on the bubble surface. The effect of nonyl-phenol added to the organic phase and to the aqueous phase can be observed in Fig. 5. From this figure it can be seen that induction time needed for bubble–drop attachment and spreading of the oil on bubble surface (Fig. 4c and d) is determined by the surfactant (nonyl-phenol) affinity in relation to the phase where it was initially dissolved. It was observed that when nonyl-phenol is initially added to the n-dodecane, the longer the stabilization time the longer was the induction time necessary for bubble–drop attachment. Considering that nonyl-phenol presents a low HLB value around 5.0, its solubility in the aqueous phase is very small. Nevertheless, its rather low adsorption at the internal surface of an oil drop strengthens the interface and lowers the spreading tendency of the drops. However, when the nonyl-phenol was added in the aqueous phase, the contact time decreased as the stabilization time increased. This is probably due to the higher affinity to the organic phase because of its low HLB value. This gives rise to a transference of nonyl-phenol molecules across the interface causing a rupture of the film at the point of contact. The attachment and spreading of the oil drop is thus favored. As far as the effect of salinity is concerned, it was observed that the induction time decreased with increasing salt concentration (Fig. 6). This can be attributed to the lowering of zeta potential of the drops by compression of the double layer, and the consequent decrease of the electrostatic repulsion.
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Fig. 8. Correlation between collision and flotation efficiencies for a n-dodecane – water – air system in absence of nonyl-phenol. .
3.3. Dissol6ed air flotation studies Preliminary work showed that both higher salinity and longer shearing time contribute to the formation of slightly smaller emulsions drops, whose diameter varied from 16 to 8 mm depending on the experimental conditions. The addition of nonyl-phenol to the aqueous phase during the preparation of the emulsion led also to the production of smaller drops, from around 15 mm in the absence of the surfactant to a value around 6 mm depending on the shearing conditions. It was observed that the presence of nonyl-phenol led to much lower flotation rates in the DAF tests, because of its effect on the collision efficiency. Fig. 7 presents the influence of the salinity on the bubble’s diameter formed in the DAF batch unit. It can be seen that bubbles formed in the absence of NaCl had about 40 mm in diameter, whereas in the presence of 3% NaCl by weight, the bubbles diameter decreased to about 25 mm.
This may be associated with the increase in surface tension by the addition of salt and has a very striking effect on the collision efficiency and consequently also in the flotation recovery of the oil, as can be seen in Fig. 8. This result is in accordance with Eqs. (2) and (3) which show that the collision efficiency is inversely proportional to approximately the square of the bubble size. On the other hand, Fig. 9 shows that the flotation efficiency is also dependent on the spreading coefficient of oil on the bubble surface and not only on the collision efficiency between bubble and drop.
4. Conclusions The results obtained from the DAF batch unit showed that the flotation efficiency depends on both hydrodynamic and physicochemical aspects. Spreading coefficient and induction time were
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135
Acknowledgements The authors acknowledge PETROBRAS and FINEP (PADCT), Brazil, for financial support.
References
Fig. 9. Correlation between flotation efficiency and spreading coefficient for emulsions prepared under different conditions.
.
shown to be useful parameters to estimate the efficiency of water cleaning by the flotation process. Experimental results showed that the induction time depends on the affinity of the surfactant for the phases involved. The presence of nonyl-phenol surfactant in the organic phase promoted not only an increase in the induction time, but also a reduction in the bubble and drop diameters. It was also observed that salinity increased the flotation efficiency by reducing the induction time and increasing collision efficiency.
[1] W.J. Trahar, Int. J. Miner. Process 8 (4) (1991) 289. [2] W.J. Trahar, J.L. Warren, Int. J. Miner. Process. 3 (2) (1976) 103. [3] G.L. Jameson, S. Nam, M. Moo Young, Miner. Sci. Eng. 9 (3) (1977) 103. [4] R.P. King, In: Principles of flotation, South African Inst. of Min. and Metallurgy, Monograph Series (1982) 215. [5] A.N. Lynch, N.W. Johnson, E.V. Manlaping, C.G. Thorne, Mineral and Coal flotation circuits, Elsevier, New York, 1981, p. 291. [6] H.I. Shulze, Physico-chemical elementary processes in flotation, Elsevier, Amsterdam, 1984, p. 238. [7] D.A. Deglon, C.T. O’Connor, Proc. of XX IMPC 3 (1997) 153. [8] W. Philippoff, Miner. Eng. 4 (1952) 386. [9] L.F. Evans, Ind. Eng. Chem. 46 (1954) 2420. [10] B.Y. Derjaguin, S.S. Dukhin, Trans. IMM 70 (1961) 221. [11] D. Reay, G.A. Ratcliff, Can. J. Chem. Eng. 51 (1975) 178. [12] L.R. Flint, W.J. Howarth, Chem. Eng. Sci. 26 (1971) 1155. [13] J. Laskowski, Miner. Sci. Eng. 6 (1974) 223. [14] G. Gonzalez, Discussions and Contributions, Trans. 87 (1978) C82. [15] G.L. Collins, G.L. Jameson, Chem. Eng. Sci. 31 (1976) 985. [16] D. Reay, G.A. Ratcliff, Can. J. Chem. Eng. 53 (5) (1975) 481. [17] J.J. Anfruns, J.A. Kitchener, Trans. IMM 86 (1977) C9. [18] W.T. Strickland Jr., J. Soc. Pet. Eng., 6 (1980) 175. [19] P.C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3rd Edition, 1997, p. 272. [20] H. Roques, Y. Aurelle, M. Aoujehane, N. Siem, J. Rabat, Revue de l’Inst. Franc¸ais du Petrole 42 (2) (1987) 163. [21] T. Brewis, Mining Magazine, July (1996) 18. [22] J. Bratby and G.V.R. Marais, In: Solid – liquid Separation Scale-up, Upland Press, Condon, 1977 158. [23] R.C.G. Oliveira, M.Sc. Thesis, COPPE/UFRJ, Federal University of Rio de Janeiro (1995).
Interfacial studies on dissolved gas flotation of oil droplets for water purification R.C.G. Oliveira a, G. Gonzalez a, J.F. Oliveira b,* a
Petrobras Research Center (CENPES), Cidade Uni6ersita´ria, Quadra 7, ZIP: 21949 -900, Rio de Janeiro, RJ Brazil b Department of Materials and Metallurgical Engineering, COPPE/UFRJ, Federal Uni6ersity of Rio de Janeiro, Cidade Uni6ersita´ria, Caixa Postal 68505, Rio de Janeiro, RJ Brazil
Abstract The removal of emulsified residual oil from water is an important issue due to the increasing concern with the environment. In the present paper various surface chemistry aspects of dissolved gas flotation were studied aiming at a better understanding of the mechanisms of the process. These studies included surface and interfacial tension measurements of a model system composed of n-dodecane in the presence and absence of nonyl-phenol. Critical micelle concentration and spreading coefficients determinations were undertaken at different NaCl concentrations. The spreading coefficient proved to be a critical parameter in oil droplets flotation. After bubble – drop contact and rupture of the water film, oil drops have a tendency to spread over the bubble surfaces. The spreading coefficient increased with increasing salt concentration. An experimental setup was designed to measure the induction contact time between single n-dodecane droplets and air bubbles. Induction time decreased with increasing salt concentration. In the presence of the nonyl-phenol aqueous solution, induction time decreased with time of stabilization. On the other hand, induction time increased with time of stabilization when nonyl-phenol was added to the organic phase. The mechanisms involved are discussed and some results obtained in a dissolved air flotation batch unit in which bubbles size was measured and controlled are also presented. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Dissolved gas flotation; Bubble–drop attachment; Flotation efficiency; Oily water cleaning
1. Introduction The effect of particle size in the flotation process has been the subject of research work in mineral processing by several authors. An excellent review has been presented by Trahar [1] in which peculiarities related to the flotation of fine, intermediate and coarse sized particles have been * Corresponding author. E-mail address: [email protected] (J.F. Oliveira)
approached independently. As far as the efficiency of collision is concerned, bubble–particle and bubble–drop contact phenomena are very similar. For this reason, most of the studies in mineral processing are also relevant to the petroleum industry. The efficiency of fine particles flotation has been the subject of some theoretical and experimental studies as indicated by reviews presented by Trahar and Warren [2], Jameson et al. [3], King [4], Linch et al. [5] and Shulze [6]. More recently, Deglon and O’Connor [7] investigated
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the floatability of fine particles in a conventional flotation cell and confirmed the strong dependence of the flotation rate constant on the particle and bubble sizes. Their results also indicated a maximum in the flotation rate constant as a function of these factors and of the pulp agitation.
1.1. Mechanisms of collision and attachment The recovery in flotation starts with the adhesion of particles to the air bubbles followed by transportation of the aggregate to the froth, drainage and enrichment of the froth and finally by its overflow removal from the cell top. However, each of these process phases occur, in fact, in several stages. The formation of a stable bubble – particle or bubble–drop aggregate is generally considered to be the rate controlling step in flotation. The first necessary condition for its effectiveness is that the mutual trajectories lead to a collision stage. Once the bubble is generally much larger than the particle, the situation is similar to that of a particle approaching a plane surface. Moreover, because of the elasticity of the bubble a depression is formed during the collision stage. Philippoff [8] and Evans [9] developed a theory to calculate the contact time based on the hypothesis that the bubble absorbs the kinetic energy of the particle during the collision. However, for flotation effectiveness it is also necessary that during this contact or induction time the thinning and rupture of the liquid film between the bubble and the drop or solid particle also occur. Derjaguin and Dukhin [10] have introduced the concept of disjoining pressure which can be applied to the bubble – particle interactions. According to this theory, when the liquid film reaches a thickness close to 0.1 mm, very strong molecular forces come into effect which could lead to the rupture of the film and the effective bubble–particle attachment. However, the shining and rupture of the film need to be followed by the expansion of the contact meniscus formed at the point of rupture which will give rise to the establishment of a contact angle large enough between the particle
and the bubble. All these stages must take place before the particle is bounced off the bubble surface when it recovers its original spherical shape.
1.2. The influence of particle and bubble sizes in flotation The attachment mechanisms briefly discussed led some authors [10–14] to formulate the flotation rate problem as a probability of flotation (Pf) according to the expression: Pf = Pc·Pa·Ps
(1)
where Pc is the probability of collision; Pa, the probability of adhesion (thinning and rupture of the liquid films during contact time) and Ps, the probability of a stable aggregate formation (a function of contact angle value). The probability of bubble–particle collision (Pc) has been directly related to physical variables such as bubble diameter, density of particles, viscosity of the pulp, relative velocity between the particle and bubble and, particularly, to the particle size. In the case of oil droplets, their low density certainly plays an important role in the collision with ascending bubbles generated by dissolved gas flotation. Reay and Ratcliff [11] studied the flotation of fine particles by dissolved gas flotation and proposed hydrodynamic equations for the movement of the particles in relation to the bubbles. They have found that the collision efficiency (Ec) is related to the diameter of the particles (dp) and that of the bubble (db) by the expression: Ec = a0
dp db
N
(2)
where Ec is the relation between the number of particles that effectively collide with a bubble and the number of particles laying along its cylindrical trajectory; a0, the proportionality factor; N, an exponent depending on the density of the particles (rp) and that of the fluid (rf); (N= 1.90 for rp/rf = 1.0 and N=2.05 for rp/rf = 2.5). Previous theoretical studies by Flint and Howarth [12] have led to a similar equation:
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Fig. 1. Schematic diagram of the dissolved gas flotation batch unit: (1) Pressure chamber; (2) Bubble generation system; (3) Flotation cell.
Ec =a1
dp db
2
(3)
where a1 is also a proportionality constant. In this last expression, the volume of each particle was disregarded and the limiting condition for collision is considered to occur when the center of the particle touches the surface of the bubble. The Reay and Ratcliff [11] hypothesis for
Fig. 3. Spreading coefficient as a function of salinity for different concentrations of nonyl-phenol.
the limiting condition, however, is that in which the particle surface just touches the bubble surface. For particles smaller than 0.2 mm, Reay and Ratcliff [11] deduced the following expression for the particle–bubble collision efficiency: Ec =
n
1.5 0.5 R2 1+ G− + R R3 (1+G)
(4)
Where R and G are given by: R= 1+
dp db
G= (R 2 − 1)·
Fig. 2. Surface tension of aqueous solutions as a function of nonyl-phenol concentration for the salinities specified.
n rp −1 rf
(5) (6)
Eqs. (3) and (4) show that for small particles the efficiency of collision increases as particle size increases and bubble size decreases. The hydrodynamic theory is based on the assumption that electrical interactions between particle and bubble are not significant. However, the theory of Derjaguin and Dukhin [10] for the rupture of the liquid film leads to the conclusion that a very strong electrical field may sometimes originate locally.
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Studies by Collins and Jameson [15] with polystyrene particles as well as those by Reay and Ratcliff [16] with glass beads showed an Ec = d 1.5 p for the case of small bubbles ( B 100 mm). Anfruns and Kitchener [17] found that Ec =d 2p for bubble sizes around 1 mm.
1.3. Beha6ior of oil drops after attachment The flotation of oil drops presents some peculiarities. First, it is important to consider that unlike solid particles the droplets are not rigid bodies and suffer deformation by the hydrodynamic effect. Their low density also has an influence in the collision stage. However, the most characteristic effect is the spreading of the oil on the bubble surface after the attachment takes place [18]. The tendency of the oil to spread as a film on the liquid – gas interface is dictated by the positive value of spreading coefficient (Sow) defined by the following expression [19]: Sow = gw −(go +gow)
(7)
where gw is the surface tension of pure water; go,
Fig. 4. Bubble–drop attachment process: (a) approach; (b) bubble – drop contact (induction time start); (c) bubble – drop attachment (induction time end); (d) interfacial film spreading.
the oil surface tension and gow, the interfacial tension between oil and water. For the case of n-dodecane, its surface tension is go = 26.1 mN m − 1 and the interfacial tension with water is gow = 41.6 mN m − 1. Thus, for gw = 71.4 mN m − 1, Sow = + 3.7 mN m − 1. This is a low value, but still leads to the spreading of the oil on the bubble surface. Fortunately, the spreading coefficient is positive for most of oil–water systems, enhancing the attachment to the bubble surface. Roques et al. [20] have studied the bubble–drop coalescence phenomena and found that the presence in the bubble of a gaseous compound transferable to the continuous aqueous phase favours the coalescence. In this way, the addition of NH3 to the air used in the flotation process considerably increased the coalescence thereby enhancing the kinetics of the process.
1.4. Inno6ations in flotation equipment An excellent review of the recent developments in flotation equipment has been presented by Brewis [21]. Concerning the flotation of oil droplets, the Jameson cell is an equipment that, due to its fast flotation kinetics and great compactness, has been used in several plant sites for the removal of organic components from hydrometallurgical liquors in SX–EW circuits [21]. This could lead to its application also in the cleaning of effluent water from the petroleum industry. However, the dispersed air flotation (DAF) has been the most widely used process for the flotation of oil drops from produced water. It consists basically of a saturator where the gas is dissolved under pressure. After introduction into the flotation cell, the contaminated water becomes supersaturated and releases the air as small bubbles. The process has been introduced for the removal of fine particles in the treatment of effluents [22]. Recently, with the worldwide environmental concern in relation to the petroleum industry, the DAF process has become widely used in the water purification stage. In the present paper, some aspects of the mechanisms of oil flotation and DAF were studied and are discussed.
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Fig. 5. Induction time as a function of the stabilization time for different nonyl-phenol concentrations added to the aqueous and organic phase.
2. Experimental
2.1. Materials and procedure The n-dodecane used was obtained from Merck (Sa˜o Paulo, Brazil) with a 99% purity. Pure nonyl-phenol, molecular weight= 225 and HLB =5.0, was supplied by Oxiteno (Rio de Janeiro, Brazil). Double distilled water, further purified by the milli-Q system, was used throughout the experiments. Its resistivity was 18.2 MV cm and surface tension around 71.5 mN m − 1 at 25°C. Other reagents used were of analytical grade. Surface tension measurements were conducted with a Kruss K10 tensiometer using the ring method. The size of the drops in the n-dodecane prepared emulsions was measured using a
Malvern 3600/E laser light scattering analyser. The determination of the initial and residual concentration of oil in water, to control the efficiency of flotation experiments, was conducted using an Horiba OCMA-220 infrared analyser.
2.2. Special apparatus To determine the bubble sizes a special rectangular cell was constructed to which the feed could be introduced via a bypass system and photographs could be taken. The optical system of a Rame´-Hart goniometer was used with a camera attachment. The determination of bubble–drop induction time needed for coalescence was carried out using a similar optical system to follow the vertical approach of a single pair of drop and bubble and
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Fig. 6. Induction time as a function of stabilization time and salinity for different NaCl concentrations with 10 − 5 M of nonyl-phenol added to the organic phase.
measure the contact time before coalescence [23]. The time of stabilization, defined as the time of immersion of both bubble and drop in the medium before contact, was also a parameter studied. The size of the bubble generated was equal to the external diameter of the capill-ary tube (3.2 mm) and that of the drop (1.4 mm). The DAF batch unit used in this work is composed of a pressure chamber, a bubble generation system and the flotation cell. The schematic diagram of the DAF batch unit is shown in Fig. 1 and details of the methods used can be found elsewhere [23].
3. Results and discussion Based on surface tension measurements, the critical micelle concentration (CMC) and spreading coefficient were determined for several experimental conditions. The induction or contact time necessary for attachment was measured as a function of the time of stabilization or immersion of drops and bubbles in the medium before contact.
3.1. Critical micelle concentration and spreading coefficient Results of surface tension measurements as a
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function of nonyl-phenol concentration are presented in Fig. 2. It can be seen that although an increase in the NaCl concentration to 3 and 6% has some effect in the surface tension at low concentration of nonyl-phenol, the CMC value remains constant around 7× 10 − 5 M. Two values of surfactant concentration lower than the CMC were thus chosen to calculate the spreading coefficient as a function of salinity (Fig. 3). In aqueous medium, the hydrocarbon droplets presented a natural tendency to adhere to the bubbles surface. Nevertheless, for the attachment to take place, the spreading coefficient should, in general, present a positive value. Because of the presence of curved interfaces, a capillary pressure contribution can, in some situations, lead to the spreading of the oil even if the spreading coefficient presents a negative value. As far as the salinity is concerned, the spreading coefficient increased slightly with increasing NaCl concentration. The effect of nonyl-phenol, however, is very striking. Fig. 3 shows that for a surfactant concentration equal to 10 − 5 M the spreading coefficient decreases to slightly negative values depending on the salinity of the water
Fig. 7. Bubble diameter as a function of nonyl-phenol concentration and salinity.
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medium. This means that the natural presence of surfactants in crude oil should be taken into consideration in the water purification by flotation, due to their decreasing effect on the spreading coefficient.
3.2. Influence of stabilization time on the induction time Fig. 4 illustrates the attachment process, which is characterized by the rupture of the aqueous film between the bubble and the drop, followed by the spreading of the oil on the bubble surface. The effect of nonyl-phenol added to the organic phase and to the aqueous phase can be observed in Fig. 5. From this figure it can be seen that induction time needed for bubble–drop attachment and spreading of the oil on bubble surface (Fig. 4c and d) is determined by the surfactant (nonyl-phenol) affinity in relation to the phase where it was initially dissolved. It was observed that when nonyl-phenol is initially added to the n-dodecane, the longer the stabilization time the longer was the induction time necessary for bubble–drop attachment. Considering that nonyl-phenol presents a low HLB value around 5.0, its solubility in the aqueous phase is very small. Nevertheless, its rather low adsorption at the internal surface of an oil drop strengthens the interface and lowers the spreading tendency of the drops. However, when the nonyl-phenol was added in the aqueous phase, the contact time decreased as the stabilization time increased. This is probably due to the higher affinity to the organic phase because of its low HLB value. This gives rise to a transference of nonyl-phenol molecules across the interface causing a rupture of the film at the point of contact. The attachment and spreading of the oil drop is thus favored. As far as the effect of salinity is concerned, it was observed that the induction time decreased with increasing salt concentration (Fig. 6). This can be attributed to the lowering of zeta potential of the drops by compression of the double layer, and the consequent decrease of the electrostatic repulsion.
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Fig. 8. Correlation between collision and flotation efficiencies for a n-dodecane – water – air system in absence of nonyl-phenol. .
3.3. Dissol6ed air flotation studies Preliminary work showed that both higher salinity and longer shearing time contribute to the formation of slightly smaller emulsions drops, whose diameter varied from 16 to 8 mm depending on the experimental conditions. The addition of nonyl-phenol to the aqueous phase during the preparation of the emulsion led also to the production of smaller drops, from around 15 mm in the absence of the surfactant to a value around 6 mm depending on the shearing conditions. It was observed that the presence of nonyl-phenol led to much lower flotation rates in the DAF tests, because of its effect on the collision efficiency. Fig. 7 presents the influence of the salinity on the bubble’s diameter formed in the DAF batch unit. It can be seen that bubbles formed in the absence of NaCl had about 40 mm in diameter, whereas in the presence of 3% NaCl by weight, the bubbles diameter decreased to about 25 mm.
This may be associated with the increase in surface tension by the addition of salt and has a very striking effect on the collision efficiency and consequently also in the flotation recovery of the oil, as can be seen in Fig. 8. This result is in accordance with Eqs. (2) and (3) which show that the collision efficiency is inversely proportional to approximately the square of the bubble size. On the other hand, Fig. 9 shows that the flotation efficiency is also dependent on the spreading coefficient of oil on the bubble surface and not only on the collision efficiency between bubble and drop.
4. Conclusions The results obtained from the DAF batch unit showed that the flotation efficiency depends on both hydrodynamic and physicochemical aspects. Spreading coefficient and induction time were
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Acknowledgements The authors acknowledge PETROBRAS and FINEP (PADCT), Brazil, for financial support.
References
Fig. 9. Correlation between flotation efficiency and spreading coefficient for emulsions prepared under different conditions.
.
shown to be useful parameters to estimate the efficiency of water cleaning by the flotation process. Experimental results showed that the induction time depends on the affinity of the surfactant for the phases involved. The presence of nonyl-phenol surfactant in the organic phase promoted not only an increase in the induction time, but also a reduction in the bubble and drop diameters. It was also observed that salinity increased the flotation efficiency by reducing the induction time and increasing collision efficiency.
[1] W.J. Trahar, Int. J. Miner. Process 8 (4) (1991) 289. [2] W.J. Trahar, J.L. Warren, Int. J. Miner. Process. 3 (2) (1976) 103. [3] G.L. Jameson, S. Nam, M. Moo Young, Miner. Sci. Eng. 9 (3) (1977) 103. [4] R.P. King, In: Principles of flotation, South African Inst. of Min. and Metallurgy, Monograph Series (1982) 215. [5] A.N. Lynch, N.W. Johnson, E.V. Manlaping, C.G. Thorne, Mineral and Coal flotation circuits, Elsevier, New York, 1981, p. 291. [6] H.I. Shulze, Physico-chemical elementary processes in flotation, Elsevier, Amsterdam, 1984, p. 238. [7] D.A. Deglon, C.T. O’Connor, Proc. of XX IMPC 3 (1997) 153. [8] W. Philippoff, Miner. Eng. 4 (1952) 386. [9] L.F. Evans, Ind. Eng. Chem. 46 (1954) 2420. [10] B.Y. Derjaguin, S.S. Dukhin, Trans. IMM 70 (1961) 221. [11] D. Reay, G.A. Ratcliff, Can. J. Chem. Eng. 51 (1975) 178. [12] L.R. Flint, W.J. Howarth, Chem. Eng. Sci. 26 (1971) 1155. [13] J. Laskowski, Miner. Sci. Eng. 6 (1974) 223. [14] G. Gonzalez, Discussions and Contributions, Trans. 87 (1978) C82. [15] G.L. Collins, G.L. Jameson, Chem. Eng. Sci. 31 (1976) 985. [16] D. Reay, G.A. Ratcliff, Can. J. Chem. Eng. 53 (5) (1975) 481. [17] J.J. Anfruns, J.A. Kitchener, Trans. IMM 86 (1977) C9. [18] W.T. Strickland Jr., J. Soc. Pet. Eng., 6 (1980) 175. [19] P.C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3rd Edition, 1997, p. 272. [20] H. Roques, Y. Aurelle, M. Aoujehane, N. Siem, J. Rabat, Revue de l’Inst. Franc¸ais du Petrole 42 (2) (1987) 163. [21] T. Brewis, Mining Magazine, July (1996) 18. [22] J. Bratby and G.V.R. Marais, In: Solid – liquid Separation Scale-up, Upland Press, Condon, 1977 158. [23] R.C.G. Oliveira, M.Sc. Thesis, COPPE/UFRJ, Federal University of Rio de Janeiro (1995).