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Gas attachment of oil droplets for gas flotation for oily wastewater cleanup
Separation and Purification Technology 33 (2003) 303 /314 www.elsevier.com/locate/seppur
Gas attachment of oil droplets for gas flotation for oily wastewater cleanup Roshni Moosai 1, Richard A Dawe * Petroleum Engineering Unit, Department of Chemical Engineering, The University of West Indies, St. Augustine, Trinidad and Tobago Received 26 September 2002; received in revised form 3 February 2003; accepted 10 February 2003
Abstract Oily wastewater cleanup can be carried out by gas flotation. When properly operated gas flotation units can reduce oil concentrations of wastewater effluents to well below 40 mg/l. Gas flotation is particularly valuable for heavy oils (oils having a density close to that of water). The flotation process relies on the attachment of gas bubbles to the dispersed oil droplets. This attachment is heavily dependent on the complex processes involving the surface characteristics of the oil droplets and their interaction with gas, and can only be optimally achieved if the surface science conditions are properly understood. The attachment mechanisms include the oil/bubble contact, the interactions of chemical additives (usually surfactants) in aiding this contact and the spreading of the oil around the gas bubble. Additionally, initial agglomeration of the oil emulsion droplets is needed to increase the droplet size to within the range needed for effective flotation, /60 mm. This paper examines the essential surface science of the gas flotation process, particularly the gas attachment to oil droplets and the use of surfactants. We discuss the stages of attachment of the gas bubble to the oil droplet, and provide further photographic evidence concerning the importance of the spreading of the oil around the gas bubble for gas flotation. # 2003 Elsevier B.V. All rights reserved. Keywords: Oily wastewater; Gas flotation; Spreading; Oil drop /gas bubble contact
1. Introduction Produced wastewater from hydrocarbon reservoirs always contains some oil, and such oily wastewater is an environmental hazard if discharged in an irresponsible manner. Much new
* Corresponding author. Fax: /868-662-4414. E-mail addresses: [email protected] Moosai), [email protected] (R.A. Dawe). 1 Now with Petrotrin, Trinidad and Tobago.
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legislation is stipulating that disposed water should contain less than 40 mg/l of oil, and this requirement is becoming more enforced as damaging environmental effects from oily wastewater become more apparent. The regulations require that non-dissolved and dissolved components are removed from the wastewater before disposal. The non-dissolved free oil in wastewater is in three forms [1]: . as drops /150 mm in diameter, which can usually be separated by conventional methods;
1383-5866/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1383-5866(03)00091-1
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. as free droplets 15/150 mm; . as a stabilised oil-in-water emulsion (often with indigenous anionic surfactants), with the median droplet diameter usually in the range of 3/ 20 mm. The dissolved form has to be removed through ion exchange or bioremediation; such processes are not discussed in this article but have been elsewhere [2,3]. The choice for any wastewater treatment facility must be tailored to the characteristics of the water to be treated, and certainly will be a compromise between factors such as space, weight, maintenance, efficiency and reliability, and importantly capital and operating costs. The industry currently prefers the gravity settler and cyclone approach to non-dissolved oil removal [4,5], but as the density difference between the oil and the water becomes smaller there is less buoyancy between the two phases, which makes these units ineffective. In particular, heavy crude oils (i.e. oils which have a density close to that of the water) are extremely difficult to treat because, in combination with properties such as high viscosities and densities and foaming characteristics, they tend to form stable emulsions with water due to the waxes, asphaltites particles and other impurities present. Gas flotation is the only effective method for cleaning non-dissolved oil in oily wastewater when the oil is heavy. 1.1. Gas flotation Gas flotation is an accelerated gravitational separation technique in which fine gas bubbles are injected into a water phase containing immiscible liquid droplets (oil) or oily solid particles so that the gas bubbles attach themselves to the droplets. The oil appears lighter because the density difference between the oil agglomerate and water is increased, consequently, the oil rises faster enabling a more rapid and effective separation from the aqueous phase. The oil droplets and oil-coated solids rise to the surface where they are trapped in the resulting foam, and removed from the flotation chamber when the foam is skimmed off [1,6,7].
This paper summarises the essential surface science of gas flotation, particularly the gas attachment to oil droplets, which involves oil/ bubble contact, the interactions of surfactants in improving this contact, and the spreading of the oil around the gas bubbles. If a full understanding of the physical processes can be gained by operators, the optimal conditions for a particular throughput and oil concentration reduction may be more successfully established, including the amount of chemicals needed. Thence operating efficiency may be increased, and costs minimised.
2. Stokes equation A major factor in gas flotation is the droplet rise velocity. The velocity of bubble/drop rise in a large volume of water has been much studied [7,8]. Solution of the Navier Stokes equation for the terminal rise velocity, V , for rigid spheres under the relevant conditions for flotation (laminar flow) gives Stokes equation, V
d 2 g(rw ro ) 18mw
where, V , droplet settling (rising or falling) velocity; d , droplet diameter; g , gravitational acceleration; rw/ro, difference in density between continuous and droplet phase (oil or gas); mw, dynamic viscosity of continuous phase. Flotation units make use of the size and density parameters in Stokes equation. Stokes equation states that the rise velocity is dependent on bubble/ droplet diameter and density difference. Oil droplet size is therefore very important; the smaller the droplets the slower the rise velocity. Attaching gas to oil reduces the oil density thereby increasing the density difference between the oil agglomerates and water and increases the agglomerate diameter thereby producing a faster rise rate. Stokes equation holds reasonably well for solid spheres in the range 10 B/d B/200 mm, [8] and for similar sized gas bubbles when surface active species are present in the water (even though the bubbles are deformable). This is because surfactants rigidify the gas/ water interface. In ‘ultrapure’ water gas bubbles
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rise faster than Stokes equation predicts, but for oily wastewater where surfactants are always present, it can be safely assumed that Stokes equation is valid [7,8]. Even so, if the oil concentration exceeds 1000 mg/l with oil droplets larger than 200 mm in diameter, a primary separator must precede any flotation unit. This is because the gas bubbles cannot float such large oil droplets, so that gas flotation cannot work effectively. In other applications increasing the gravitational acceleration by centrifuges is carried out [4].
3. Gas flotation of oily wastewater Gas bubbles and oil drops must contact, then attach, for flotation to occur. As oil and gas are both less dense than water, (except for some very heavy oils), they will both rise if placed in water. To achieve a rate of rise which enables reasonable residence times in a separation unit (B/30 min) one has to increase the droplet size (comes into Stokes equation as a squared power), or increase the difference in fluid densities (e.g. gas floatation, as described in this paper). Additionally it is advantageous to decrease the viscosity of the continuous phase (e.g. by increasing fluid temperature).
3.1. Oil droplet size The oil in oilfield wastewater is often mainly stabilised oil-in-water emulsion with the droplets in the range 3/20 mm [1,9,10]. Estimates using Stokes equation predict that it will take around 50 s for a 70 mm, 600 s for 20 mm and 3000 s for 10 mm diameter oil drop for a 10 mm rise in an aqueous medium for an oil /water density difference of 0.1 g/cc [9]. Thus we need the average diameter of the oil /gas agglomerate to be greater than 60 mm to satisfy the residence time constraint, as flotation units are commonly 2 m high. The water is strongly saline, often with salt concentrations / 20 000 mg/l, but sometimes over 100 000 mg/l. Such salinities affect the charge on the droplets (Sections 4.1 and 8).
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3.2. Gas bubble contacts Gas bubbles are generally larger than oil drops, often being /50 mm. Due to this larger gas bubble size and the larger density difference between the gas and water ( /0.9 g/cm3 compared to oil and water of /0.1 g/cm3, or smaller for heavy oil where ro :/rw), gas bubbles will generally rise some 10 /100 times faster compared to oil drops of similar diameters, if rising in a true vertical manner and following Stokes equation, and so overtake the oil drops. This can lead to bubble /droplet contact. A range of gas bubbles is beneficial because the smaller bubbles can capture the smaller oil droplets and the larger ones the larger droplets. Naturally too, the longer the residence time of the gas bubbles in the flotation tanks, the greater the number of gas bubble-oil droplet collisions (contact efficiency), the greater the quantity of the oil that ought to be removed, but clearly water throughput is also an important consideration. 3.3. Other parameters The effectiveness of gas flotation of oily wastewater depends not only on the traditional gravity separation parameters within Stokes equation of liquid density difference, oil droplet size and distribution, temperature and viscosity as discussed above, but also on gas bubble size and bubble size distribution and degree of dispersion, inlet concentration of oil and its variability, chemical content of the wastewater and the oil, pH and viscosity of the aqueous phase, and the interfacial properties between the oil, gas and brine, particularly interfacial tensions, wettability and spreading coefficients [1,11]. Chemicals can be added to assist the attachment as discussed later in Section 5 [12,13]. If there is such chemical intervention, the type of chemicals added, and their concentration are important. Sizing of equipment also needs knowledge of flowrate, gas-input rate and volume of gas per unit volume of wastewater, correct dosage of chemicals and an appropriate skimming device to remove the froth layer containing the oil at the surface (this froth can itself become a new additional waste hazard) [6,7].
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3.4. Oil /gas bubble hydrodynamics The bubble and drop must first come into close proximity so that their mutual trajectories lead to collision, Fig. 1. The motion normally observed within the flotation chamber is erratic and has a complex flow pattern, with complicated flow streamlines of the oil droplets and gas bubbles with many of the oil drops deflecting past the gas bubbles rather than making the desired collision and attachment, [1,7,14] making collision frequency difficult to estimate. Such calculations are made even more difficult because the gas bubbles grow as they ascend due to a reduction of hydrostatic pressure during bubble rise (about 0.3% for a 100 mm rise), and because gas can diffuse into the bubble from the water if the water is supersaturated with gas. Hydrodynamic theories of the collision between particles and bubbles have been developed, with that by Reay and Ratcliff generally accepted as being elegant and satisfactory [1,7,11,15]. They showed that gas flotation is most effective when the oil droplets have diameters between 3 and 100 mm and that the efficiency is not greatly affected by bubble size, but is significantly affected by bubble number density. Thus they suggest it is better to have oil drops as large as possible (larger collision area) and bubble size as small as possible (longer residence times). Even so, even if the hydrodynamics of the system are perfect and the gas bubbles and oil droplets contact as shown in
Fig. 1. The oil/ gas bubble rise hydrodynamics.
Figs. 1 and 2a, and create gas/oil agglomerates, a strong adhesion of the gas bubble to the oil drop is paramount as discussed later, otherwise, additional collisions during the upwards motion of the agglomerates will break them up. 3.5. Gas bubble introduction In field practice there are two major methods of introducing the gas bubbles */those of induced (sometimes termed dispersed) gas flotation, IGF, and of dissolved gas flotation, DGF [6,7]. The significant differences between the two flotation processes are the average bubble size, the mixing conditions and the hydraulic loading rating. Induced flotation has the higher values [6]. There are other more sophisticated (and expensive) methods for bubble generation such as electrolytic gas bubble generation, which is sometimes used in municipal wastewater systems, but not for oilfield gas flotation [7]. 3.5.1. Induced gas flotation Induced gas flotation is where gas is drawn into the flotation chamber through a special type of disperser such as revolving impellers or ejectors and is used on some oil production installations, especially offshore [1,6]. Gas bubbles are normally in the range of 1000 mm. Retention time in the treatment unit can be as low as 4 min. 3.5.2. Dissolved gas flotation Dissolved gas flotation is where water is saturated with gas under pressure (but usually no more than four atmospheres), and then passed into the flotation chamber. The pressure is reduced to atmospheric at the inlet to the flotation chamber, which leads to the release of gas bubbles with diameters in the range of 20/100 mm, with a median /60 mm. Retention time in the flotation chamber is usually about 15 /30 min and is a fairly gentle process [6,7]. The design of DGF units has undergone extensive modifications to improve effectiveness [16 / 22]. For instance designs have included multicell systems incorporating coalescing media, creating microfine bubbles using a spinner design inside the vessel [20], having a hydrocyclone in the inlet
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Fig. 2. The attachment process. (a) Bubble and drop approach; (b) water film thinning; (c) thin film dimples due to interfacial tension gradients; (d) the dimple disappears as film drains and thins further as detailed in Fig. 3; (e) at a critical thickness film ruptures, and if spreading conditions are present, the oil spreads around the gas; (f) the conglomerate then continues to rise. If these processes have not occurred within the time frame of approach, the bubble and drop do not attach but move away from each other.
piping removing solids and the larger oil drops before they enter the flotation zone, adjustable weirs which remove free oil entering the flotation section of the vessel and diverter baffles to effect some coalescence of drops [20], others have even incorporated a jet pump [21] and a multi-stage loop [22]. 3.5.3. Various gases Various gases have been used for the flotation process. Field gas (mainly methane) is frequently used on oil production installations but can be explosive and methane gas is a serious greenhouse effect hazard if released to the atmosphere, being 20 times more potent than carbon dioxide. Inert gas (sometimes air) may be used. Air increases the oxygen content of the discharged water but iron may be precipitated. Additionally, air can oxidise the oil creating a sticky mass and air combined with gas mixtures can be a safety risk due to explosions.
4. Factors affecting flotation efficiency Flotation is achieved by enabling oil drops to attach themselves to gas bubbles, so that the
increased density differential makes the oil rise faster to the surface of the wastewater. Flotation is dependent on [7]: . hydrodynamic forces (e.g. the movements of the bubbles, drops and continuous phase) . thermodynamic forces (e.g. interfacial interactions) . physicochemical aspects (e.g. chemical interactions affecting the interfacial interactions). The efficiency of separation can be increased by coalescence of oil drops, which may be aided by surfactants and/or chemical demulsifiers. The attachment of oil drops to gas bubbles and the formation of a stable bubble drop aggregate are rate-controlling steps. The essential stages in the flotation process are, Fig. 2: . demulsification of oily wastewater and increase of oil droplet size by coalescence . approach of oil drops and gas bubbles . drainage and rupture of the interstitial film . attachment of gas bubbles to the oil with, for successful flotation, spreading of oil drops onto gas bubbles. Clearly during this short period (a few micro to perhaps a millisecond), the surface
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forces that give attachment and the drainage of the liquid film must be complete. . rise of the coalesced phases to the surface for it to be skimmed off. Experimental and theoretical studies have been individually reported in the surface science literature for all these processes, but still the prediction of gas flotation efficiency from knowledge of basic physical parameters is insufficient [23 /33]. 4.1. Elastic behaviour and surface charges The interfacial interactions between gas bubbles and oil droplets create the attachment of the bubble to the oil surface which are counteracted by the elastic behaviour in collisions of both the bubble and the drop. Due to this elasticity, contacts can lead to just deformation of surfaces with subsequent rebound, rather than attachment. Additionally, both interfaces normally have similar charge, although recent studies suggest that the charge on the gas bubbles maybe small for strongly saline solutions, as is often found in oilfield wastewaters [34]. The oil drops usually have negative charges on their surfaces due to electrostatic forces created by the electric double layers, creating a repulsive force between the drops, which keep them apart. This means they have a low probability of coalescing in this state and chemicals are needed to neutralise these charges, as discussed below.
5. Chemical intervention for demulsification, coalescence and bubble attachment As stated in Section 3.1, the oil in wastewater is usually in the form of oil in water emulsion, with most droplet diameters being less than 20 mm with a median commonly as low as 5 mm [9,10]. This means that the small droplet rise times are far too long to allow separation by gravity as a deoiling process for industrial oily wastewater purposes, where one lets the wastewater stand. Gas flotation is then essential but before flotation can be effective industrially, the emulsion must be destabilised and the oil droplets grown by coalescence
to at least 60 mm. Clearly it is necessary to increase d in Stokes equation of the oil drops by flocculating the oil droplets into larger sized agglomerates. In effective oily wastewater treatment, chemical treatment with surface-active agents (surfactants) is usually carried out to achieve this flocculation as will be discussed below [6,12]. The first stage is to reduce the negative charges present, particularly on the oil drop surfaces. If these charges are not neutralised, the subsequent repulsions between the oil drops hinder oil droplet enlargement through emulsion destabilisation, which in turn would reduce gas bubble-oil droplet attachment. 5.1. Demulsifiers and flocculants The wastewater requires demulsification to enlarge the small oil droplets. This can be achieved by chemical, electrolytic or physical methods. In gas flotation, chemicals are the norm with the addition of organic ionic polymers as demulsifiers and coalescers. The added chemical (surfactant) must enable the drop and bubble to approach each other and then remain sufficiently close and long enough to establish drainage and rupture to allow oil to spread over the gas. The purpose of any chemical intervention is to modify the charges between the oil droplets and the oil /gas interfaces. This is achieved by introducing an opposite charge, which encourages the oil droplets to flocculate and to assist in the attachment of the gas to the oil drops. If the surfactants do not perform this job (e.g. if a suitable concentration of surfactant is not used) spreading will not occur and the gas bubbles will rebound from the oil drops. 5.2. Surfactant properties Surfactants, or surface active agents, are molecules which are able to modify the properties of an interface, e.g. liquid/air or liquid/liquid by lowering the surface or interfacial tension. A surfactant possesses the fundamental characteristic of having two essential portions, one being water repellent, usually called hydrophobic (or oleophilic), the other being water attractive, usually called hydrophilic (or oleophobic). The hydrophobic
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portion comprises a collection of hydrocarbon groups, some at least of which form a linear chain which may or may not be substituted to varying extents. The hydrophilic portion comprises a solubilising group such as sulphate, sulphonate or ethoxylate. Surfactants fall into four categories depending on the distribution of electrical charge on the molecule [18,23] viz. . anionic in which the hydrophobic portion of the molecule carries a residual negative charge, RCOO X e.g. sodium dodecyl sulphate: CH3CH2(CH2)9CH2OSO3Na . cationic in which the hydrophobic portion carries a residual positive charge, R X e.g. cetyltrimethyl ammonium chloride: CH3(CH2)14CH2 /N(CH3)3/Cl . non-ionic in which there is no residual electrical charge, e.g. dodecylalcohol ethoxylate: CH3(CH2)10CH2(OCH2CH2)n OH . amphoteric in which both positive and negative centres; are to be found in the molecule, e.g. alkyldimethylbetaine: (CH3)/,(CH3)/,R /N / CH2COO. The number and arrangement of the hydrocarbon groups together with the nature and position of the hydrophilic groups combine to determine the surface active properties of the molecule. If the hydrophilic portion is high molecular weight, it is termed a polymer surfactant. 5.3. Surfactant use The surfactant molecules orient themselves at the oil /water interface. Currently in oily wastewater industrial practice cationic and anionic surfactants and polymers are used to tailor the floc size, floating characteristics and shear strength. These polymers have a long chain and in addition to charge neutralisation enables mechanical bridging of oil drops to create flocs. A larger rising velocity will occur according to Stokes equation when the radius of the ‘oil drop’ increases through such floc formation. In practice the chemicals used to give optimum results typically would be a primary coagulant (emulsion breaker) being a low molecular weight,
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cationic (i.e. with the hydrophobic part carrying a residual positive charge) polymer which is able to neutralise the negative charge on the oil droplet as it adsorbs on the surface of the oil droplet. They are the primary neutraliser and added first and operate through mechanical bridging in addition to charge neutralisation. A flocculant, which is a high molecular weight (perhaps 2000) cationic or anionic polymer, is added after to create macroflocs where a number of oil drops become attached to one another creating a larger oil drop. The flocs will also increase the collision rate with the gas bubbles in the wastewater, further increasing the chances of oil/bubble attachment and possibly form even larger oil /gas flocs. The preferred size of the polymer molecule is such that, several oil globules can become attached to one polymer, which causes them to coalesce into larger particles by a bridging mechanism. It is found that such organic emulsion breakers, rather than inorganic demulsifiers, often require less dosage and produce a smaller volume of sludge, in addition to producing better effluent quality [12]. In terms of physical interactions, the demulsifier breaks emulsions . by modifying the charge on the oil droplets (usually negative), and . causes flocculation by anchorage and bridging mechanisms [12,23 /25]. This is followed by a high molecular weight (/ 2000) anionic polymer, which can then be used to promote growth of the floc through mechanical bridging. Unfortunately, information on flocculant and surfactant composition used commercially is not often divulged in product literature, much to the disadvantage of those wishing to select their own optimum flocculants and minimise their costs. Experiments, such as bottle (sometimes called jar) tests or small scale flotation apparatus, must be carried out to identify suitable chemicals (surfactants) and their concentrations. Bottle tests involve the mixing of the various chemicals with samples of the contaminated water, shaking and observing the results. Such tests can eliminate some chemicals and identify others which might
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be effective under the field conditions. However a problem can sometimes occur when scaling-up the laboratory test results for application to field operations, because a surfactant’s effectiveness is highly dependent particularly on the Critical Micelle Concentration, CMC, which is affected by the field water characteristics, which can vary with time of day and season and wastewater composition. The CMC is the concentration of surfactant that is needed to form a monolayer on the oil surfaces, and above this value aggregates of surfactant molecule, termed micelles, begin to be formed [23]. Hence the field inlet wastewater stream must be monitored closely, especially if surges in concentration occur frequently. Any deviations from the conditions of the bottle testing programme can have detrimental effects on the flotation system. 5.4. Critical micelle concentration effects The maximum amount of surfactants that should be added is the CMC, for the particular salinity of the water [23]. Above this concentration, the surfactant concentration is ‘overshot’ and causes the droplets to gain increased positive charge and repel each other so that the separation process loses its efficiency. If too little surfactant is used, flotation will be less efficient since the conditions are not optimum (perhaps 20% or more less effective). Hence before chemicals are added, its optimum concentration (CMC) must be determined [23]. This value will vary according to the field conditions including diurnal temperature variations, concentration of wastewater, oil properties, and the salinity of water. Bottle tests are therefore essential.
6. Layer thinning 6.1. Attachment of oil drops to oil drops (flocculation) and of oil drops with gas bubbles When two drops (oil drop/gas bubble or oil/oil drops) approach each other, a thin layer (often described as a thin film) of water is created, Fig. 2a and b. In this paper we shall discuss the thinning
and rupture of the gas /oil interface. The explanations follow similar lines as those in emulsion breaking [24 /31]. Further approach causes liquid to be squeezed out of the thinning film: the drainage of the film. The surfactants have a significant role here and must be soluble in the water phase otherwise they cannot be dispersed. As the liquid drains a characteristic dimple is created, which induces a pressure distribution in the film, Fig. 2c [24]. A concentration gradient between the centre region of the film and its interfaces occurs which creates fluid movement through Gibbs /Marangoni effects [25 /31]. 6.2. Gibbs /Marangoni effects The surfactant concentration varies along the thin film (Fig. 2dFig. 3), causing a reduction in interfacial tension leading to an interfacial tension gradient along the surface so producing a force, a Gibbs /Marangoni effect [25 /29]. The concentration of solute in the bulk of the film (centre region) will be high but will have a low concentration adsorbed at the interfaces because of the geometry of the gas bubble-oil drop set up and radial movement of the draining liquid (Fig. 3). A small increase in interfacial tension occurs as the surfactant (small quantities) spreads over the interfaces creating an incomplete monolayer. In the case of a bubble and oil or two drops of oil very strong intermolecular forces come into effect as the film thins. Such forces can lead to rupture of the film. Drainage proceeds relatively slowly until the surfactants have redistributed along the interfaces so that the concentration at the centre gives an interfacial tension difference that just balances at any particular thickness (capillary pressures). Further approach causes more drainage of the film by fluid outflow due to these capillary pressure gradients, which are in turn due to surfactant concentration gradients (again a Marangoni effect). The dimple gradually disappears making the water film thin [25,26]. This interstitial film drains under the action of capillary suction balanced by disjoining pressure, where disjoining pressure consists of van der Waals dispersion forces and electrostatic forces. Water-soluble cationic flocculants create the high capillary pressure
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Fig. 3. Detail of the attachment process. (a) Water film thinning; (b) thin film dimples due to interfacial tension gradients; (c) as film drains and thus thins further the dimple disappears; (d) the film ruptures at a critical thickness.
and cause the instability of the thin film (B/80 nm). It has been shown that flocculants can deplete this interfacial film with the driving force being an osmotic pressure gradient, and water in the thin laminar layer is drawn out [25 /31]. Gravity forces only play a minor role in this process [28,31].
6.3. Oil/bubble attachment through rupture The main movement within the fluid film occurs in the radial direction with velocity varying with position in the film. The surfaces of the films are mobile and have surfactants within the interfaces [25,26]. The concentration of surfactants along the oil/water and gas/water surfaces at the centre falls due to surface expansion caused by the dimpling. When the liquid film drops to a critical thickness of approximately 0.10 mm, the disjoining pressure dominates and very strong intermolecular forces come into effect, the laminar layer becomes unstable and ruptures very quickly [25 /31]. The lifetime of the film is determined by the rate at which drainage takes place, but all this has to happen in the short period (less than a few milliseconds) and whilst the gas bubble and oil drop are close together. Upon rupture, coalescence between the gas bubble-oil drop or two oil drops occurs.
6.4. Spreading Spreading of the oil over the bubble then has to occur, otherwise the oil /gas agglomerate will not be sufficiently robust to withstand the upward rise because the agglomerate will just be connecting at points [32,33,35]. Fluid/fluid interactions are usually described by the spreading coefficients. The spreading coefficient of a fluid, S , (defined below) is the imbalance between the interfacial tensions (forces) acting along a single line (contact line between fluid phases) [35,36]. For the gas / oil /water system, the oil spreading coefficient on a water /gas interface can be defined as [1,35/38]: So gwg gow gog where gwg is the water /gas surface tension, gow is the oil /water interfacial tension and gog is the oil / gas surface tension. A spreading coefficient can be either positive or negative; if it is positive it indicates that one of the tensions is larger than the sum of the other two and leads to the total spreading of a fluid layer onto the interface; if it is negative (non-spreading) the fluid will form a definite contact angle with the other two phases. For effective gas flotation So must be positive and gwg is larger than the sum of other two interfacial tensions (gwo/gog), so that oil tends to form a spreading continuous film on the water /gas interface. Gravitational and/or viscous forces may
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affect the spreading process but only at a secondary level. The velocity of spreading is proportional to the spreading coefficient and decreases as the oil viscosity increases. The velocity of spreading for many polar oils on gas /water is about 1 /10 cm/s [38]. 6.4.1. An example of spreading oil on gas An example of spreading oil on gas is shown in Fig. 4 taken from experiments performed to visually observe gas and oil behaviour for enhanced oil recovery, where the physics is very similar to that for successful oil flotation in water. This beautiful picture was taken using micromodels and compliments others [35,38 /41]. The positive value of So (/2.5 dynes/cm) means that whenever the three phases of gas, oil, and water come into contact, the oil phase always forms a continuous film between the gas and water. Spreading ensures the attachment of the oil to the bubble is maintained while it rises to the surface. For a gas bubble of 50 mm diameter, an oil drop with a diameter of 50 mm will form a layer of 13
mm, 20 mm will form a layer around the bubble of around 1 mm and for a drop of diameter 10 mm, a film of 0.15 mm will be formed. Floating oil droplets less than 10 mm is therefore not generally successful as a very thin unstable film around the bubble would be formed. Thus another reason to grow the oil droplets to over 40 mm, preferably larger, by coalescence before attempting to float them. If the system is non-oil spreading, the adherence of the oil to the gas bubble is weak and the agglomerate is likely to break-up as it rises. Unfortunately, the ideal surfactant to achieve the specific purpose of enhanced spreading is still uncertain.
7. Rise The oil in the wastewater now with the attached gas bubbles rises to the top of the flotation chamber to form a froth, and is removed from the cell top by skimming of the overflow, Fig. 2f, and is finally disposed of in some environmentally acceptable way, perhaps biodegradation.
8. Effect of salt concentration
Fig. 4. Gas/oil/water configuration for spreading oil spreading conditions (So //2.5 dynes/cm). The oil is spread around the bubble.
Salt concentration influences the flotation of oil drops in saline water by modifying surface charges on gas and oil drops (Section 4.1), as well as affecting the gas bubble size, the CMC and spreading coefficients [1,23,32,33]. From Stokes equation the parameters that affect the flotation process are the radii of oil droplets, bulk phase viscosity and the density differential between the oil and aqueous phases all of which are affected, albeit not always strongly, by the salinity. In addition, salt concentration has been shown to have an effect on the oil drop and gas /oil interactions as well as on the interfacial tension, so that the spreading of oil on the gas and the attachment to gas bubbles is also affected. Salt concentration directly influences the collision efficiency of oil drops and gas bubbles and therefore the flotation efficiency. Clearly too the salt concentration can affect ionic surfactants by modify-
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ing the charges on the hydrophobic part of the molecules and exchange of the ionic species. This again emphasises that tests such as bottle tests have to be carried out under field conditions.
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Acknowledgements We are grateful to Dr C. Grattoni, Imperial College, London, UK and R. Gayoso Transep SA Argentina for much helpful advice and to The Campus Research and Publication Fund Committee, UWI, for financial assistance.
9. Conclusions The efficiency of gas flotation depends particularly on gas bubble size and number. When properly designed and operated, oil concentrations of effluents from gas flotation units can generally be kept to well below 40 mg/l, but chemical additives are normally essential for efficient operation of the gas flotation process. Flotation works well with drops 20 B/d B/150 mm, but has little effect when the droplets are less than 3 mm. It is clear that the controlling factor in the successful flotation of the oil in wastewater is gas bubble/oil droplet attachment. The process of spreading of the oil on the surface of the gas bubbles is paramount. The actual processes of gas /oil attachment are complicated. First the formation of a thick film lamella by drop and bubble approach, followed by the drainage (thinning of the lamella) to a thin film, which then ruptures. The driving force behind the drainage of the film is capillary pressure and the thinning rate is governed by the hydrodynamic and thermodynamic interactions. Spreading occurs when the interfacial tensions give a positive oil /gas spreading coefficient. This is critical for effective flotation of the oil droplets. Growing the bubble directly on the oil surface from a supersaturated solution could solve this problem. This could be achieved if the gas nuclei were on the oil drop surface, which entails having solid particles on the oil drop surface which could be sandstone particles or more likely long-chain polymers adsorbed onto the surface. Further knowledge of the chemical intervention is needed before gas flotation can be fully reliable and economical under the variable ‘dirty’ conditions in the field.
References [1] W.T. Strickland, Laboratory results of cleaning produced water by gas flotation, SPE 7805, Soc. Pet. Eng. J., 10 (1980) 175 /190. [2] B.R. Hansen, S.R.H. Davies, Review of potential technologies for the removal of dissolved components from produced water, Trans. IChemE, Chem. Eng. Res. Des. 72 (Part A) (1994) 176 /188. [3] A. Seybold, J. Cook, R.V. Rajan, R. Hickey, J. Miller, A. Lawrence, T. Hayes, Demonstration of dissolved organics removal from produced water, SPE 37420, Production Operations Symposium, Oklahoma, March, 1997. [4] K.E. Arnold, M. Stewart, Surface production operations */Design of Oil Handling Systems and Facilities, vol. 1, second ed., Gulf Publishing Co, Houston, Texas, 1998. [5] Z.I. Khatib, Handling, treatment and disposal of produced water in the offshore oil industry, SPE 48992, Annual Technical Conference, New Orleans, 1998. [6] R. Moosai, R.A. Dawe, Oily wastewater cleanup by gas flotation’’, West Indian J. Eng. 25 (1) (2002) 25 /41. [7] J.A. Kitchener, The froth flotation process: past, present and future */in brief, In: K.J. Ives (Ed.), The Scientific Basis of Flotation, NATO ASI Series no 75, 1984, pp. 3 / 51 (Chapter 1 and the other chapters). [8] G.J. Jameson, Physics and hydrodynamics of bubbles, In: K.J. Ives (Ed.), The Scientific Basis of Flotation, NATO ASI Series no 75, 1984, pp. 53 /77 (Chapter 2). [9] A. Finborud, M. Faucher, E. Sellman, New method for improving oil droplet growth for separation enhancement, SPE 56643, Annual Technical Conference, Houston, TX, Oct, 1999. [10] M.J. van der Zande, P.H. Janssen, W.M.G.T. van den Broek, Size of oil droplets under high water-cut condition, SPE 67250, SPE Production and Operations Symposium, Oklahoma, March, 2001. [11] N.D. Sylvester, J.J. Byeseda, Oil/water separation by induced air flotation, SPE 7886, Soc. Pet. Eng. J. 10 (1980) 579 /590. [12] X. Zhu, B.E. Reed, W. Lin, P.E. Carriere, G. Roark, Investigation of emulsified oil wastewater treatment with polymers, Separation Purification Technol. 32 (13) (1997) 2173 /2187. [13] B.W. Bradley, Flotation offers another water/oil separation technique, Oil Gas J. Dec. 9 (1985) 43 /45.
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[14] T. Dabros, H.A. Hamza, D. Sai, J. Czarnecki, Attachment of gas bubbles to solid surfaces from an impinging jet, in: K.K. Mittal, P. Kumar (Eds.), Emulsions, Foams and Thin Films (ISBN 0-8247-036-9, Chapter 24), Marcel Dekker, New York, 2000, pp. 411 /431 (ISBN 0-8247036-9, Chapter 24). [15] C. Reay, G.A. Ratcliff, Removal of fine particles from water by dispersed air flotation: effects of bubble size on collection efficiency, Can. J. Chem. Eng. 51 (1973) 178 / 185. [16] J.A. Ramirez, R.H. Davis, Microflotation of fine oil droplets by small air bubbles: experiment and theory, Separation Purification Technol. 36 (1) (2001) 1 /15. [17] M.C. Sport, Design and operation of dissolved-gas flotation equipment for the treatment of oilfield produced brines, SPE 2717, J. Pet. Tech. Aug (1970) 918 /920. [18] R.C. Shaw, J. Czarnecki, L.L. Schramm, D. Axelson, Bituminous froths in the hot-water flotation process, In: L.L. Schramm (Ed.), Emulsions: Fundamentals and Applications in the Petroleum Industry, Advances in Chemistry Series 242, American Chemical Society, Washington, DC, 1994, pp. 423 /459 (ISBN 0-8412-2719-5, Chapter 11). [19] A.L. Cassidy, Advances in flotation unit design for produced water treatment, SPE 25472, Production Operations Symposium, Oklahoma, March, 1993. [20] Monosep Corporation, Louisiana, USA; available from: http://www.monosep.com. [21] Z. Youyi, Z. Qiang, N. Yabin, Development of a new high effective flotation device used for water treatment, SPE 65373, International Symposium on Oilfield Chemistry, Houston, TX, Feb, 2001. [22] X. Gu, S. Chiang, A novel flotation column for oily water clean up, Separation Purification Technol. 16 (1999) 193 / 203. [23] A. Patist, S.-G. Oh, S.Y. Shiao, T.-F. Ling, H.K. Lee, M.K. Sharma, S. Devi, D.O. Shah, Unity in diversity in interfacial phenomena, in: K.K. Mittal, P. Kumar (Eds.), Emulsions, Foams and Thin Films (ISBN 0-8247-036-9, Chapter 2), Marcel Dekker, New York, 2000, pp. 31 /57 (ISBN 0-8247-036-9, Chapter 2). [24] D. Platikanov, Experimental investigation on the ‘dimpling’ of thin liquid films, J. Phys. Chem. 68 (12) (1964) 3619 /3624. [25] A.D. Nikolov, M. Randie, C.S. Shetty, D.T. Wasan, Chemical demulsification of oil-in-water emulsion using air-flotation: the importance of film thickness stability, Chem. Eng. Comm. 152 /153 (1996) 337 /350. [26] D.T. Wasan, K. Koczo, A.D. Nikolov, Mechanisms of aqueous foam stability and antifoaming action with and without oil, in: L.L. Schramm (Ed.), Emulsions: Fundamentals and Applications in the Petroleum Industry, Advances in Chemistry Series 242 (ISBN 0-8412-2719-5, Chapter 2), American Chemical Society, Washington, DC, 1994, pp. 47 /114 (ISBN 0-8412-2719-5, Chapter 2).
[27] P.A. Kralchevsky, K.D. Danov, I.B. Ivanov, Thin liquid film physics, in: R.K. Prud’homme, S.A. Khan (Eds.), Foams, Theory, Measurements and Applications (ISBN 08247-9395-1, Chapter 1), Marcel Dekker, New York, 1996, pp. 1 /98 (ISBN 0-8247-9395-1, Chapter 1). [28] J.C. Lee, T.D. Hodgson, Film flow and coalescence */I. Basic relations, film shape and criteria for interface mobility, Chem. Eng. Sci. 23 (1968) 1375 /1397. [29] A.K. Chesters, The modelling of coalescence processes in fluid-liquid dispersions */A review of current understanding, Trans. IChemE 69 (Part A) (1991) 259 /270. [30] G. Narsimhan, E. Ruckenstein, Structure, drainage and coalescence of foams and concentrated emulsion, in: R.K. Prud’homme, S.A. Khan (Eds.), Foams, Theory, Measurements and Applications (ISBN 0-8247-9395-1, Chapter 2), Marcel Dekker, New York, 1996, pp. 99 /187 (ISBN 08247-9395-1, Chapter 2). [31] S. Simpson, R.K. Jain, V. Raghuraman, P. Somasundaran, Thinning and rupture of aqueous surfactant films on silica, in: K.K. Mittal, P. Kumar (Eds.), Emulsions, Foams and Thin Films (ISBN 0-8247-036-9, Chapter 13), Marcel Dekker, New York, 2000, pp. 233 /250 (ISBN 0-8247036-9, Chapter 13). [32] R.C.G. Oliveira, G. Gonzalez, J.F. Oliveira, Interfacial studies on dissolved gas flotation of oil droplets for water purification, Colloids Surf. A 154 (1999) 127 /135. [33] R.C.G. Oliveira, G. Gonzalez, J.F. Oliveira, Interfacial studies on dissolved gas flotation, SPE 50767, International Symposium on Oilfield Chemistry, Houston, TX, 1999. [34] C. Yang, T. Dabros, D. Li, J. Czarnecki, J.H. Masliyah, Measurement of the zeta potential of gas bubbles in aqueous solutions by microelectrophoresis method, J. Colloid Interface Sci. 243 (2001) 128 /135. [35] C.A. Grattoni, R. Moosai, R.A. Dawe, Photographic observations showing spreading and non-spreading of oil on gas bubbles of relevance to gas flotation for oily wastewater cleanup, Colloids Surf. A: Physicochem. Eng. Aspects, in press. [36] G.J. Hirasaki, Wettability: fundamentals and surface forces, SPE 17367, SPE Formation Evaluation June (1991) 217 /226. [37] A.W. Adamson, Physical Chemistry of Surfaces, fifth ed., Wiley-Interscience, New York, 1990. [38] J.T. Davies, E.K. Rideal, Interfacial Phenomena, second ed., Academic Press, New York, 1963, pp. 25 /28. [39] R.I. Hawes, R.A. Dawe, R.N. Evans, The release of solution gas from waterflooded residual oil, Soc. Pet. Eng. J. 2(Dec) (1997) 379 /388. [40] R.A. Dawe, C.A. Grattoni, The visualisation of the pore scale physics of hydrocarbon recovery from reservoirs, First Break 16(Nov) (1998) 368 /371. [41] R.A. Dawe, R.I. Hawes, C.A. Grattoni, Production of gas from waterflood residual oil /gas saturation build-up and relative permeabilities, Petrol. Geosci. 7 (2001) 131 /136.
Gas attachment of oil droplets for gas flotation for oily wastewater cleanup Roshni Moosai 1, Richard A Dawe * Petroleum Engineering Unit, Department of Chemical Engineering, The University of West Indies, St. Augustine, Trinidad and Tobago Received 26 September 2002; received in revised form 3 February 2003; accepted 10 February 2003
Abstract Oily wastewater cleanup can be carried out by gas flotation. When properly operated gas flotation units can reduce oil concentrations of wastewater effluents to well below 40 mg/l. Gas flotation is particularly valuable for heavy oils (oils having a density close to that of water). The flotation process relies on the attachment of gas bubbles to the dispersed oil droplets. This attachment is heavily dependent on the complex processes involving the surface characteristics of the oil droplets and their interaction with gas, and can only be optimally achieved if the surface science conditions are properly understood. The attachment mechanisms include the oil/bubble contact, the interactions of chemical additives (usually surfactants) in aiding this contact and the spreading of the oil around the gas bubble. Additionally, initial agglomeration of the oil emulsion droplets is needed to increase the droplet size to within the range needed for effective flotation, /60 mm. This paper examines the essential surface science of the gas flotation process, particularly the gas attachment to oil droplets and the use of surfactants. We discuss the stages of attachment of the gas bubble to the oil droplet, and provide further photographic evidence concerning the importance of the spreading of the oil around the gas bubble for gas flotation. # 2003 Elsevier B.V. All rights reserved. Keywords: Oily wastewater; Gas flotation; Spreading; Oil drop /gas bubble contact
1. Introduction Produced wastewater from hydrocarbon reservoirs always contains some oil, and such oily wastewater is an environmental hazard if discharged in an irresponsible manner. Much new
* Corresponding author. Fax: /868-662-4414. E-mail addresses: [email protected] Moosai), [email protected] (R.A. Dawe). 1 Now with Petrotrin, Trinidad and Tobago.
(R.
legislation is stipulating that disposed water should contain less than 40 mg/l of oil, and this requirement is becoming more enforced as damaging environmental effects from oily wastewater become more apparent. The regulations require that non-dissolved and dissolved components are removed from the wastewater before disposal. The non-dissolved free oil in wastewater is in three forms [1]: . as drops /150 mm in diameter, which can usually be separated by conventional methods;
1383-5866/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1383-5866(03)00091-1
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. as free droplets 15/150 mm; . as a stabilised oil-in-water emulsion (often with indigenous anionic surfactants), with the median droplet diameter usually in the range of 3/ 20 mm. The dissolved form has to be removed through ion exchange or bioremediation; such processes are not discussed in this article but have been elsewhere [2,3]. The choice for any wastewater treatment facility must be tailored to the characteristics of the water to be treated, and certainly will be a compromise between factors such as space, weight, maintenance, efficiency and reliability, and importantly capital and operating costs. The industry currently prefers the gravity settler and cyclone approach to non-dissolved oil removal [4,5], but as the density difference between the oil and the water becomes smaller there is less buoyancy between the two phases, which makes these units ineffective. In particular, heavy crude oils (i.e. oils which have a density close to that of the water) are extremely difficult to treat because, in combination with properties such as high viscosities and densities and foaming characteristics, they tend to form stable emulsions with water due to the waxes, asphaltites particles and other impurities present. Gas flotation is the only effective method for cleaning non-dissolved oil in oily wastewater when the oil is heavy. 1.1. Gas flotation Gas flotation is an accelerated gravitational separation technique in which fine gas bubbles are injected into a water phase containing immiscible liquid droplets (oil) or oily solid particles so that the gas bubbles attach themselves to the droplets. The oil appears lighter because the density difference between the oil agglomerate and water is increased, consequently, the oil rises faster enabling a more rapid and effective separation from the aqueous phase. The oil droplets and oil-coated solids rise to the surface where they are trapped in the resulting foam, and removed from the flotation chamber when the foam is skimmed off [1,6,7].
This paper summarises the essential surface science of gas flotation, particularly the gas attachment to oil droplets, which involves oil/ bubble contact, the interactions of surfactants in improving this contact, and the spreading of the oil around the gas bubbles. If a full understanding of the physical processes can be gained by operators, the optimal conditions for a particular throughput and oil concentration reduction may be more successfully established, including the amount of chemicals needed. Thence operating efficiency may be increased, and costs minimised.
2. Stokes equation A major factor in gas flotation is the droplet rise velocity. The velocity of bubble/drop rise in a large volume of water has been much studied [7,8]. Solution of the Navier Stokes equation for the terminal rise velocity, V , for rigid spheres under the relevant conditions for flotation (laminar flow) gives Stokes equation, V
d 2 g(rw ro ) 18mw
where, V , droplet settling (rising or falling) velocity; d , droplet diameter; g , gravitational acceleration; rw/ro, difference in density between continuous and droplet phase (oil or gas); mw, dynamic viscosity of continuous phase. Flotation units make use of the size and density parameters in Stokes equation. Stokes equation states that the rise velocity is dependent on bubble/ droplet diameter and density difference. Oil droplet size is therefore very important; the smaller the droplets the slower the rise velocity. Attaching gas to oil reduces the oil density thereby increasing the density difference between the oil agglomerates and water and increases the agglomerate diameter thereby producing a faster rise rate. Stokes equation holds reasonably well for solid spheres in the range 10 B/d B/200 mm, [8] and for similar sized gas bubbles when surface active species are present in the water (even though the bubbles are deformable). This is because surfactants rigidify the gas/ water interface. In ‘ultrapure’ water gas bubbles
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rise faster than Stokes equation predicts, but for oily wastewater where surfactants are always present, it can be safely assumed that Stokes equation is valid [7,8]. Even so, if the oil concentration exceeds 1000 mg/l with oil droplets larger than 200 mm in diameter, a primary separator must precede any flotation unit. This is because the gas bubbles cannot float such large oil droplets, so that gas flotation cannot work effectively. In other applications increasing the gravitational acceleration by centrifuges is carried out [4].
3. Gas flotation of oily wastewater Gas bubbles and oil drops must contact, then attach, for flotation to occur. As oil and gas are both less dense than water, (except for some very heavy oils), they will both rise if placed in water. To achieve a rate of rise which enables reasonable residence times in a separation unit (B/30 min) one has to increase the droplet size (comes into Stokes equation as a squared power), or increase the difference in fluid densities (e.g. gas floatation, as described in this paper). Additionally it is advantageous to decrease the viscosity of the continuous phase (e.g. by increasing fluid temperature).
3.1. Oil droplet size The oil in oilfield wastewater is often mainly stabilised oil-in-water emulsion with the droplets in the range 3/20 mm [1,9,10]. Estimates using Stokes equation predict that it will take around 50 s for a 70 mm, 600 s for 20 mm and 3000 s for 10 mm diameter oil drop for a 10 mm rise in an aqueous medium for an oil /water density difference of 0.1 g/cc [9]. Thus we need the average diameter of the oil /gas agglomerate to be greater than 60 mm to satisfy the residence time constraint, as flotation units are commonly 2 m high. The water is strongly saline, often with salt concentrations / 20 000 mg/l, but sometimes over 100 000 mg/l. Such salinities affect the charge on the droplets (Sections 4.1 and 8).
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3.2. Gas bubble contacts Gas bubbles are generally larger than oil drops, often being /50 mm. Due to this larger gas bubble size and the larger density difference between the gas and water ( /0.9 g/cm3 compared to oil and water of /0.1 g/cm3, or smaller for heavy oil where ro :/rw), gas bubbles will generally rise some 10 /100 times faster compared to oil drops of similar diameters, if rising in a true vertical manner and following Stokes equation, and so overtake the oil drops. This can lead to bubble /droplet contact. A range of gas bubbles is beneficial because the smaller bubbles can capture the smaller oil droplets and the larger ones the larger droplets. Naturally too, the longer the residence time of the gas bubbles in the flotation tanks, the greater the number of gas bubble-oil droplet collisions (contact efficiency), the greater the quantity of the oil that ought to be removed, but clearly water throughput is also an important consideration. 3.3. Other parameters The effectiveness of gas flotation of oily wastewater depends not only on the traditional gravity separation parameters within Stokes equation of liquid density difference, oil droplet size and distribution, temperature and viscosity as discussed above, but also on gas bubble size and bubble size distribution and degree of dispersion, inlet concentration of oil and its variability, chemical content of the wastewater and the oil, pH and viscosity of the aqueous phase, and the interfacial properties between the oil, gas and brine, particularly interfacial tensions, wettability and spreading coefficients [1,11]. Chemicals can be added to assist the attachment as discussed later in Section 5 [12,13]. If there is such chemical intervention, the type of chemicals added, and their concentration are important. Sizing of equipment also needs knowledge of flowrate, gas-input rate and volume of gas per unit volume of wastewater, correct dosage of chemicals and an appropriate skimming device to remove the froth layer containing the oil at the surface (this froth can itself become a new additional waste hazard) [6,7].
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3.4. Oil /gas bubble hydrodynamics The bubble and drop must first come into close proximity so that their mutual trajectories lead to collision, Fig. 1. The motion normally observed within the flotation chamber is erratic and has a complex flow pattern, with complicated flow streamlines of the oil droplets and gas bubbles with many of the oil drops deflecting past the gas bubbles rather than making the desired collision and attachment, [1,7,14] making collision frequency difficult to estimate. Such calculations are made even more difficult because the gas bubbles grow as they ascend due to a reduction of hydrostatic pressure during bubble rise (about 0.3% for a 100 mm rise), and because gas can diffuse into the bubble from the water if the water is supersaturated with gas. Hydrodynamic theories of the collision between particles and bubbles have been developed, with that by Reay and Ratcliff generally accepted as being elegant and satisfactory [1,7,11,15]. They showed that gas flotation is most effective when the oil droplets have diameters between 3 and 100 mm and that the efficiency is not greatly affected by bubble size, but is significantly affected by bubble number density. Thus they suggest it is better to have oil drops as large as possible (larger collision area) and bubble size as small as possible (longer residence times). Even so, even if the hydrodynamics of the system are perfect and the gas bubbles and oil droplets contact as shown in
Fig. 1. The oil/ gas bubble rise hydrodynamics.
Figs. 1 and 2a, and create gas/oil agglomerates, a strong adhesion of the gas bubble to the oil drop is paramount as discussed later, otherwise, additional collisions during the upwards motion of the agglomerates will break them up. 3.5. Gas bubble introduction In field practice there are two major methods of introducing the gas bubbles */those of induced (sometimes termed dispersed) gas flotation, IGF, and of dissolved gas flotation, DGF [6,7]. The significant differences between the two flotation processes are the average bubble size, the mixing conditions and the hydraulic loading rating. Induced flotation has the higher values [6]. There are other more sophisticated (and expensive) methods for bubble generation such as electrolytic gas bubble generation, which is sometimes used in municipal wastewater systems, but not for oilfield gas flotation [7]. 3.5.1. Induced gas flotation Induced gas flotation is where gas is drawn into the flotation chamber through a special type of disperser such as revolving impellers or ejectors and is used on some oil production installations, especially offshore [1,6]. Gas bubbles are normally in the range of 1000 mm. Retention time in the treatment unit can be as low as 4 min. 3.5.2. Dissolved gas flotation Dissolved gas flotation is where water is saturated with gas under pressure (but usually no more than four atmospheres), and then passed into the flotation chamber. The pressure is reduced to atmospheric at the inlet to the flotation chamber, which leads to the release of gas bubbles with diameters in the range of 20/100 mm, with a median /60 mm. Retention time in the flotation chamber is usually about 15 /30 min and is a fairly gentle process [6,7]. The design of DGF units has undergone extensive modifications to improve effectiveness [16 / 22]. For instance designs have included multicell systems incorporating coalescing media, creating microfine bubbles using a spinner design inside the vessel [20], having a hydrocyclone in the inlet
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Fig. 2. The attachment process. (a) Bubble and drop approach; (b) water film thinning; (c) thin film dimples due to interfacial tension gradients; (d) the dimple disappears as film drains and thins further as detailed in Fig. 3; (e) at a critical thickness film ruptures, and if spreading conditions are present, the oil spreads around the gas; (f) the conglomerate then continues to rise. If these processes have not occurred within the time frame of approach, the bubble and drop do not attach but move away from each other.
piping removing solids and the larger oil drops before they enter the flotation zone, adjustable weirs which remove free oil entering the flotation section of the vessel and diverter baffles to effect some coalescence of drops [20], others have even incorporated a jet pump [21] and a multi-stage loop [22]. 3.5.3. Various gases Various gases have been used for the flotation process. Field gas (mainly methane) is frequently used on oil production installations but can be explosive and methane gas is a serious greenhouse effect hazard if released to the atmosphere, being 20 times more potent than carbon dioxide. Inert gas (sometimes air) may be used. Air increases the oxygen content of the discharged water but iron may be precipitated. Additionally, air can oxidise the oil creating a sticky mass and air combined with gas mixtures can be a safety risk due to explosions.
4. Factors affecting flotation efficiency Flotation is achieved by enabling oil drops to attach themselves to gas bubbles, so that the
increased density differential makes the oil rise faster to the surface of the wastewater. Flotation is dependent on [7]: . hydrodynamic forces (e.g. the movements of the bubbles, drops and continuous phase) . thermodynamic forces (e.g. interfacial interactions) . physicochemical aspects (e.g. chemical interactions affecting the interfacial interactions). The efficiency of separation can be increased by coalescence of oil drops, which may be aided by surfactants and/or chemical demulsifiers. The attachment of oil drops to gas bubbles and the formation of a stable bubble drop aggregate are rate-controlling steps. The essential stages in the flotation process are, Fig. 2: . demulsification of oily wastewater and increase of oil droplet size by coalescence . approach of oil drops and gas bubbles . drainage and rupture of the interstitial film . attachment of gas bubbles to the oil with, for successful flotation, spreading of oil drops onto gas bubbles. Clearly during this short period (a few micro to perhaps a millisecond), the surface
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forces that give attachment and the drainage of the liquid film must be complete. . rise of the coalesced phases to the surface for it to be skimmed off. Experimental and theoretical studies have been individually reported in the surface science literature for all these processes, but still the prediction of gas flotation efficiency from knowledge of basic physical parameters is insufficient [23 /33]. 4.1. Elastic behaviour and surface charges The interfacial interactions between gas bubbles and oil droplets create the attachment of the bubble to the oil surface which are counteracted by the elastic behaviour in collisions of both the bubble and the drop. Due to this elasticity, contacts can lead to just deformation of surfaces with subsequent rebound, rather than attachment. Additionally, both interfaces normally have similar charge, although recent studies suggest that the charge on the gas bubbles maybe small for strongly saline solutions, as is often found in oilfield wastewaters [34]. The oil drops usually have negative charges on their surfaces due to electrostatic forces created by the electric double layers, creating a repulsive force between the drops, which keep them apart. This means they have a low probability of coalescing in this state and chemicals are needed to neutralise these charges, as discussed below.
5. Chemical intervention for demulsification, coalescence and bubble attachment As stated in Section 3.1, the oil in wastewater is usually in the form of oil in water emulsion, with most droplet diameters being less than 20 mm with a median commonly as low as 5 mm [9,10]. This means that the small droplet rise times are far too long to allow separation by gravity as a deoiling process for industrial oily wastewater purposes, where one lets the wastewater stand. Gas flotation is then essential but before flotation can be effective industrially, the emulsion must be destabilised and the oil droplets grown by coalescence
to at least 60 mm. Clearly it is necessary to increase d in Stokes equation of the oil drops by flocculating the oil droplets into larger sized agglomerates. In effective oily wastewater treatment, chemical treatment with surface-active agents (surfactants) is usually carried out to achieve this flocculation as will be discussed below [6,12]. The first stage is to reduce the negative charges present, particularly on the oil drop surfaces. If these charges are not neutralised, the subsequent repulsions between the oil drops hinder oil droplet enlargement through emulsion destabilisation, which in turn would reduce gas bubble-oil droplet attachment. 5.1. Demulsifiers and flocculants The wastewater requires demulsification to enlarge the small oil droplets. This can be achieved by chemical, electrolytic or physical methods. In gas flotation, chemicals are the norm with the addition of organic ionic polymers as demulsifiers and coalescers. The added chemical (surfactant) must enable the drop and bubble to approach each other and then remain sufficiently close and long enough to establish drainage and rupture to allow oil to spread over the gas. The purpose of any chemical intervention is to modify the charges between the oil droplets and the oil /gas interfaces. This is achieved by introducing an opposite charge, which encourages the oil droplets to flocculate and to assist in the attachment of the gas to the oil drops. If the surfactants do not perform this job (e.g. if a suitable concentration of surfactant is not used) spreading will not occur and the gas bubbles will rebound from the oil drops. 5.2. Surfactant properties Surfactants, or surface active agents, are molecules which are able to modify the properties of an interface, e.g. liquid/air or liquid/liquid by lowering the surface or interfacial tension. A surfactant possesses the fundamental characteristic of having two essential portions, one being water repellent, usually called hydrophobic (or oleophilic), the other being water attractive, usually called hydrophilic (or oleophobic). The hydrophobic
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portion comprises a collection of hydrocarbon groups, some at least of which form a linear chain which may or may not be substituted to varying extents. The hydrophilic portion comprises a solubilising group such as sulphate, sulphonate or ethoxylate. Surfactants fall into four categories depending on the distribution of electrical charge on the molecule [18,23] viz. . anionic in which the hydrophobic portion of the molecule carries a residual negative charge, RCOO X e.g. sodium dodecyl sulphate: CH3CH2(CH2)9CH2OSO3Na . cationic in which the hydrophobic portion carries a residual positive charge, R X e.g. cetyltrimethyl ammonium chloride: CH3(CH2)14CH2 /N(CH3)3/Cl . non-ionic in which there is no residual electrical charge, e.g. dodecylalcohol ethoxylate: CH3(CH2)10CH2(OCH2CH2)n OH . amphoteric in which both positive and negative centres; are to be found in the molecule, e.g. alkyldimethylbetaine: (CH3)/,(CH3)/,R /N / CH2COO. The number and arrangement of the hydrocarbon groups together with the nature and position of the hydrophilic groups combine to determine the surface active properties of the molecule. If the hydrophilic portion is high molecular weight, it is termed a polymer surfactant. 5.3. Surfactant use The surfactant molecules orient themselves at the oil /water interface. Currently in oily wastewater industrial practice cationic and anionic surfactants and polymers are used to tailor the floc size, floating characteristics and shear strength. These polymers have a long chain and in addition to charge neutralisation enables mechanical bridging of oil drops to create flocs. A larger rising velocity will occur according to Stokes equation when the radius of the ‘oil drop’ increases through such floc formation. In practice the chemicals used to give optimum results typically would be a primary coagulant (emulsion breaker) being a low molecular weight,
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cationic (i.e. with the hydrophobic part carrying a residual positive charge) polymer which is able to neutralise the negative charge on the oil droplet as it adsorbs on the surface of the oil droplet. They are the primary neutraliser and added first and operate through mechanical bridging in addition to charge neutralisation. A flocculant, which is a high molecular weight (perhaps 2000) cationic or anionic polymer, is added after to create macroflocs where a number of oil drops become attached to one another creating a larger oil drop. The flocs will also increase the collision rate with the gas bubbles in the wastewater, further increasing the chances of oil/bubble attachment and possibly form even larger oil /gas flocs. The preferred size of the polymer molecule is such that, several oil globules can become attached to one polymer, which causes them to coalesce into larger particles by a bridging mechanism. It is found that such organic emulsion breakers, rather than inorganic demulsifiers, often require less dosage and produce a smaller volume of sludge, in addition to producing better effluent quality [12]. In terms of physical interactions, the demulsifier breaks emulsions . by modifying the charge on the oil droplets (usually negative), and . causes flocculation by anchorage and bridging mechanisms [12,23 /25]. This is followed by a high molecular weight (/ 2000) anionic polymer, which can then be used to promote growth of the floc through mechanical bridging. Unfortunately, information on flocculant and surfactant composition used commercially is not often divulged in product literature, much to the disadvantage of those wishing to select their own optimum flocculants and minimise their costs. Experiments, such as bottle (sometimes called jar) tests or small scale flotation apparatus, must be carried out to identify suitable chemicals (surfactants) and their concentrations. Bottle tests involve the mixing of the various chemicals with samples of the contaminated water, shaking and observing the results. Such tests can eliminate some chemicals and identify others which might
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be effective under the field conditions. However a problem can sometimes occur when scaling-up the laboratory test results for application to field operations, because a surfactant’s effectiveness is highly dependent particularly on the Critical Micelle Concentration, CMC, which is affected by the field water characteristics, which can vary with time of day and season and wastewater composition. The CMC is the concentration of surfactant that is needed to form a monolayer on the oil surfaces, and above this value aggregates of surfactant molecule, termed micelles, begin to be formed [23]. Hence the field inlet wastewater stream must be monitored closely, especially if surges in concentration occur frequently. Any deviations from the conditions of the bottle testing programme can have detrimental effects on the flotation system. 5.4. Critical micelle concentration effects The maximum amount of surfactants that should be added is the CMC, for the particular salinity of the water [23]. Above this concentration, the surfactant concentration is ‘overshot’ and causes the droplets to gain increased positive charge and repel each other so that the separation process loses its efficiency. If too little surfactant is used, flotation will be less efficient since the conditions are not optimum (perhaps 20% or more less effective). Hence before chemicals are added, its optimum concentration (CMC) must be determined [23]. This value will vary according to the field conditions including diurnal temperature variations, concentration of wastewater, oil properties, and the salinity of water. Bottle tests are therefore essential.
6. Layer thinning 6.1. Attachment of oil drops to oil drops (flocculation) and of oil drops with gas bubbles When two drops (oil drop/gas bubble or oil/oil drops) approach each other, a thin layer (often described as a thin film) of water is created, Fig. 2a and b. In this paper we shall discuss the thinning
and rupture of the gas /oil interface. The explanations follow similar lines as those in emulsion breaking [24 /31]. Further approach causes liquid to be squeezed out of the thinning film: the drainage of the film. The surfactants have a significant role here and must be soluble in the water phase otherwise they cannot be dispersed. As the liquid drains a characteristic dimple is created, which induces a pressure distribution in the film, Fig. 2c [24]. A concentration gradient between the centre region of the film and its interfaces occurs which creates fluid movement through Gibbs /Marangoni effects [25 /31]. 6.2. Gibbs /Marangoni effects The surfactant concentration varies along the thin film (Fig. 2dFig. 3), causing a reduction in interfacial tension leading to an interfacial tension gradient along the surface so producing a force, a Gibbs /Marangoni effect [25 /29]. The concentration of solute in the bulk of the film (centre region) will be high but will have a low concentration adsorbed at the interfaces because of the geometry of the gas bubble-oil drop set up and radial movement of the draining liquid (Fig. 3). A small increase in interfacial tension occurs as the surfactant (small quantities) spreads over the interfaces creating an incomplete monolayer. In the case of a bubble and oil or two drops of oil very strong intermolecular forces come into effect as the film thins. Such forces can lead to rupture of the film. Drainage proceeds relatively slowly until the surfactants have redistributed along the interfaces so that the concentration at the centre gives an interfacial tension difference that just balances at any particular thickness (capillary pressures). Further approach causes more drainage of the film by fluid outflow due to these capillary pressure gradients, which are in turn due to surfactant concentration gradients (again a Marangoni effect). The dimple gradually disappears making the water film thin [25,26]. This interstitial film drains under the action of capillary suction balanced by disjoining pressure, where disjoining pressure consists of van der Waals dispersion forces and electrostatic forces. Water-soluble cationic flocculants create the high capillary pressure
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Fig. 3. Detail of the attachment process. (a) Water film thinning; (b) thin film dimples due to interfacial tension gradients; (c) as film drains and thus thins further the dimple disappears; (d) the film ruptures at a critical thickness.
and cause the instability of the thin film (B/80 nm). It has been shown that flocculants can deplete this interfacial film with the driving force being an osmotic pressure gradient, and water in the thin laminar layer is drawn out [25 /31]. Gravity forces only play a minor role in this process [28,31].
6.3. Oil/bubble attachment through rupture The main movement within the fluid film occurs in the radial direction with velocity varying with position in the film. The surfaces of the films are mobile and have surfactants within the interfaces [25,26]. The concentration of surfactants along the oil/water and gas/water surfaces at the centre falls due to surface expansion caused by the dimpling. When the liquid film drops to a critical thickness of approximately 0.10 mm, the disjoining pressure dominates and very strong intermolecular forces come into effect, the laminar layer becomes unstable and ruptures very quickly [25 /31]. The lifetime of the film is determined by the rate at which drainage takes place, but all this has to happen in the short period (less than a few milliseconds) and whilst the gas bubble and oil drop are close together. Upon rupture, coalescence between the gas bubble-oil drop or two oil drops occurs.
6.4. Spreading Spreading of the oil over the bubble then has to occur, otherwise the oil /gas agglomerate will not be sufficiently robust to withstand the upward rise because the agglomerate will just be connecting at points [32,33,35]. Fluid/fluid interactions are usually described by the spreading coefficients. The spreading coefficient of a fluid, S , (defined below) is the imbalance between the interfacial tensions (forces) acting along a single line (contact line between fluid phases) [35,36]. For the gas / oil /water system, the oil spreading coefficient on a water /gas interface can be defined as [1,35/38]: So gwg gow gog where gwg is the water /gas surface tension, gow is the oil /water interfacial tension and gog is the oil / gas surface tension. A spreading coefficient can be either positive or negative; if it is positive it indicates that one of the tensions is larger than the sum of the other two and leads to the total spreading of a fluid layer onto the interface; if it is negative (non-spreading) the fluid will form a definite contact angle with the other two phases. For effective gas flotation So must be positive and gwg is larger than the sum of other two interfacial tensions (gwo/gog), so that oil tends to form a spreading continuous film on the water /gas interface. Gravitational and/or viscous forces may
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affect the spreading process but only at a secondary level. The velocity of spreading is proportional to the spreading coefficient and decreases as the oil viscosity increases. The velocity of spreading for many polar oils on gas /water is about 1 /10 cm/s [38]. 6.4.1. An example of spreading oil on gas An example of spreading oil on gas is shown in Fig. 4 taken from experiments performed to visually observe gas and oil behaviour for enhanced oil recovery, where the physics is very similar to that for successful oil flotation in water. This beautiful picture was taken using micromodels and compliments others [35,38 /41]. The positive value of So (/2.5 dynes/cm) means that whenever the three phases of gas, oil, and water come into contact, the oil phase always forms a continuous film between the gas and water. Spreading ensures the attachment of the oil to the bubble is maintained while it rises to the surface. For a gas bubble of 50 mm diameter, an oil drop with a diameter of 50 mm will form a layer of 13
mm, 20 mm will form a layer around the bubble of around 1 mm and for a drop of diameter 10 mm, a film of 0.15 mm will be formed. Floating oil droplets less than 10 mm is therefore not generally successful as a very thin unstable film around the bubble would be formed. Thus another reason to grow the oil droplets to over 40 mm, preferably larger, by coalescence before attempting to float them. If the system is non-oil spreading, the adherence of the oil to the gas bubble is weak and the agglomerate is likely to break-up as it rises. Unfortunately, the ideal surfactant to achieve the specific purpose of enhanced spreading is still uncertain.
7. Rise The oil in the wastewater now with the attached gas bubbles rises to the top of the flotation chamber to form a froth, and is removed from the cell top by skimming of the overflow, Fig. 2f, and is finally disposed of in some environmentally acceptable way, perhaps biodegradation.
8. Effect of salt concentration
Fig. 4. Gas/oil/water configuration for spreading oil spreading conditions (So //2.5 dynes/cm). The oil is spread around the bubble.
Salt concentration influences the flotation of oil drops in saline water by modifying surface charges on gas and oil drops (Section 4.1), as well as affecting the gas bubble size, the CMC and spreading coefficients [1,23,32,33]. From Stokes equation the parameters that affect the flotation process are the radii of oil droplets, bulk phase viscosity and the density differential between the oil and aqueous phases all of which are affected, albeit not always strongly, by the salinity. In addition, salt concentration has been shown to have an effect on the oil drop and gas /oil interactions as well as on the interfacial tension, so that the spreading of oil on the gas and the attachment to gas bubbles is also affected. Salt concentration directly influences the collision efficiency of oil drops and gas bubbles and therefore the flotation efficiency. Clearly too the salt concentration can affect ionic surfactants by modify-
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ing the charges on the hydrophobic part of the molecules and exchange of the ionic species. This again emphasises that tests such as bottle tests have to be carried out under field conditions.
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Acknowledgements We are grateful to Dr C. Grattoni, Imperial College, London, UK and R. Gayoso Transep SA Argentina for much helpful advice and to The Campus Research and Publication Fund Committee, UWI, for financial assistance.
9. Conclusions The efficiency of gas flotation depends particularly on gas bubble size and number. When properly designed and operated, oil concentrations of effluents from gas flotation units can generally be kept to well below 40 mg/l, but chemical additives are normally essential for efficient operation of the gas flotation process. Flotation works well with drops 20 B/d B/150 mm, but has little effect when the droplets are less than 3 mm. It is clear that the controlling factor in the successful flotation of the oil in wastewater is gas bubble/oil droplet attachment. The process of spreading of the oil on the surface of the gas bubbles is paramount. The actual processes of gas /oil attachment are complicated. First the formation of a thick film lamella by drop and bubble approach, followed by the drainage (thinning of the lamella) to a thin film, which then ruptures. The driving force behind the drainage of the film is capillary pressure and the thinning rate is governed by the hydrodynamic and thermodynamic interactions. Spreading occurs when the interfacial tensions give a positive oil /gas spreading coefficient. This is critical for effective flotation of the oil droplets. Growing the bubble directly on the oil surface from a supersaturated solution could solve this problem. This could be achieved if the gas nuclei were on the oil drop surface, which entails having solid particles on the oil drop surface which could be sandstone particles or more likely long-chain polymers adsorbed onto the surface. Further knowledge of the chemical intervention is needed before gas flotation can be fully reliable and economical under the variable ‘dirty’ conditions in the field.
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