Modified Jet Flotation in Oil (Petroleum) Emulsion/Water Separations

Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244

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Modiﬁed jet ﬂotation in oil (petroleum) emulsion/water separations

M. Santander 1, R.T. Rodrigues, J. Rubio ∗

Laboratório de Tecnologia Mineral e Ambiental, Departamento de Engenharia de Minas, PPG3M, Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonc¸ alves 9500, Prédio 75, Porto Alegre, RS 91501-970, Brazil article info abstract

Article history: This work presents results of a rapid emulsified oil (petroleum) removal from water by flocculation Received 20 August 2010 followed by flotation in a modified jet (Jameson) cell (MJC). The modification is such that the downcomer Received in revised form 2 December 2010 was sealed at the bottom (by a concentric blind-end tube) to allow floated particles to enter immediately Accepted 10 December 2010 into the frothy phase after the capture of the oily flocs by the bubbles. Also, a packed bed (crowder) Available online 23 December 2010 was placed at the upper part of the concentric tube to stabilize the froth and facilitate the rise of the oil floc/bubble aggregates. The work was divided into two parts: a detailed laboratory study (1.3 m3/h) and a Keywords: pilot plant trial in an offshore platform. Parameters studied were flocculation (type and concentration of Jet flotation Oily pollutants polymer), oil concentration, oil droplets size distribution and flotation cell design. Results of laboratory Petroleum separation studies showed mean separation efficiencies of the order of 80% when used as a conventional jet cell Platform (CJC) with feed emulsions (droplets size of about 20 ␮m) ranged between 100 and 400 mg/L petroleum concentration. The oil removal increased up to 85% in the MJC. These studies allowed optimizing the design and process parameters: chemical, physico-chemical and operating. A MJC (5 m3/h) was then projected, built and installed in an offshore platform, after the oil extraction–production point. At optimal conditions, in a single flotation stage, discharges varied between 20 and 30 mg/L oil concentration or 81% removal at 24.7 m3/h m2 loading capacity. Because this jet cell operates with a high air hold-up, it presented a very good efficiency (capture of oil droplets by bubbles) at low residence time (high-rate separation) and showed to be simple, compact and easy to operate. It is believed that the MJC has a great potential for treating polluted oily high flow wastewaters, at high separation rate. Results and mechanisms involved are discussed in terms of interfacial phenomena and design factors. © 2011 Elsevier B.V. All rights reserved.

1. Introduction commonly discharged into the ocean environment and may cause severe environmental petroleum contamination especially when Mining, metallurgical, petroleum and chemical industries gen- reaching surface, ground and coastal waterways. erate huge amounts of wastewater usually polluted by solids, Therefore treatment of these effluents is required and must process chemicals, organic and other compounds [1–4]. Table 1 result in improved oil/water rapid separation, improved water summarizes main oil/organic sources reported. quality, oil recovery, water reuse, amongst others. The conventional During crude oil exploration and production large volumes of technology for the treatment of oily produced water on offshore petroleum hydrocarbon bearing effluents, the so-called produced platforms usually includes a degasser (to remove the natural gas waters, are concurrently recovered. These waters usually contain that accompanies oil) and oil–water separators (mainly gravity set- high salinity, suspended solids (clay, sand, scale corrosion prod- tlers). ucts), total dissolved solids and the oil may range between 100 and Many techniques for separation of oil–water emulsions are 1000 mg/L or still higher depending on oil separation process effi- available, namely filters [5], ultra-filtration [6], micro-filtration [7], ciency and nature of crude oil. Crude oils are a complex mixture reverse osmosis, gravity separation, activated sludge treatment of many hydrocarbons which vary in their toxicity to aquatic and [7], dissolved air flotation [8,9], column flotation [10], flotation terrestrial life. These produced waters (after treatment) are more with gas-aphrons [11], electroflotation [12], induced air flotation [13], membrane bioreactor [14], carbon adsorption, chemical coagulation, electrocoagulation. The advantages and disadvantages of these processes have been already fully discussed by Bande et al. ∗ Corresponding author. Tel.: +55 51 33089479; fax: +55 51 33089477. [12]. E-mail address: [email protected] (J. Rubio). The efficiency of these techniques depends on feed concentra- URL: http://www.ufrgs.br/ltm (J. Rubio). 1 Permanent address: Departamento de Metalurgia, CRIDESAT, Universidad de tion but mainly on the form of oil in the aqueous phase; if disperse Atacama, Av. Copayapu N◦ 485, Copiapó, Región de Atacama, Chile. (oil droplets > 50 ␮m), emulsified (oil droplets < 50 ␮m) and dis-

0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.12.027 238 M. Santander et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244

Table 1 a feed line and forms a liquid jet. Air is entrained into the liq- Main organic pollutants in different industrial activities. uid in the downcomer by a vacuum effect and sheared into Petroleum exploration and oil Hydrocarbons, alcohols, ethers, phenols, many bubbles. Thus a very good environment for particle collec- refineries carbon disulfide, sulfonic acids, etc. tion by bubbles (air hold-up >40%) is created in the downcomer Beneficiation of agates and Minerals oils, dyes, diesel oil, etc. [42,43]. amethysts Treatment of ores and Organic solvents of solvent extraction This cell has shown a great potential, nor only in mineral pro- metallurgy processes flotation reagents: foaming, cessing but also for solid/liquid separations and for liquid/liquid collectors and surface modifiers separations [40,42]. Its main advantage is its high rate process effi- Process metallurgy Cutting oils, solvents ciency and moderate equipment cost [41,44]. Chemical and petrochemical Various oils and fats, organic reagents, Problems with process efficiency have been recently solved surfactants with low-shear mixing head, recycle of treated water and use of polymeric flocculants in the downcomer. This allowed its use in wastewater treatment and recovery of solvent extraction liquors solved. The selection process for the treatment of oily produced [37], municipal waters [45], treatment of wastes from a variety of water oil platforms offshore will depend also on the equipment industries, such as dairy factories, abattoirs, metal finishing, rolling foot-print and performance because the space of the platform is mills, coke ovens. reduced. Advantages recognized, amongst others, are: Yet, the flotation efficiency will be determined mainly by the degree of emulsion destabilization stage. Poor performance has • Compact design and low capital cost; always been observed when flocculation was incomplete. The flota- • More, with no moving parts, the jet cell has low power con- tion of organic rich waters such as oil spills on water, oily sewage sumption and low maintenance costs: (air self-entrained, no or oil-in-water emulsions has been used for a number of decades compressor or blower required); in various fields. • Improved performance in a number of specific areas: mineral pro- Because of collection and adhesion problems, the separation of cessing, solvent extraction, industrial (treatment of effluents) and the very fine oil droplets (<50 ␮m) by flotation requires fine bub- municipal wastewater industries; bles, quiescent hydrodynamic conditions in the cell and emulsion • Low residence times (<3 min), high throughput and high effi- breakers prior to flotation [15]. ciency. Table 2 summarizes most of the flotation processes applied to oily effluents including organic liquors and solvents. In the CJC modification to the basic design [46], for effluents 1.1. The Jameson flotation cell – background containing fragile flocs, the feed containing suspended particu- lates is mixed with a bubbly flow generated by a plunging jet The cell (originally from the mineral processing field) consists of recycled clean liquid, in a relatively quiescent zone in the of an aeration/contact zone (the downcomer), a bubble-particle downcomer. or aggregate disengagement zone (the tank proper pulp area) The flotation separation of very fine oil droplets (2–30 ␮m) is and a cleaning or froth forming zone (the tank proper zone). In even more complicated and usually requires fine bubbles, qui- contrast to conventional mechanical flotation units, the Jameson escent hydrodynamic conditions in the cell separation zone or cell was designed to accomplish fast flotation based on a high emulsion breakers prior to flotation [32]. This is due to collection bubble surface area flux, as a result of the very many fine bub- and adhesion factors, which makes the process very slow, espe- bles generated by high shear rate in the downcomer. The bubbles cially when, treating high flow-rates. IAF (induced air flotation) and (medium size) formed in this cell may have 100–600 ␮m in diam- DAF, have been used extensively in the removal of stable oily emul- eter [40,41]. sions [15,20,23]. IAF utilizes bubbles between 40 and 1000 ␮min In a Jameson cell, the mineral feed (or wastewater) and the size and turbulent hydrodynamic conditions. The process has low air are introduced at the top and travel downwards in the down- retention times, normally <5 min. Conversely, DAF employs micro- comer which is essentially a vertical pipe. The pulp enters through bubbles (30–100 ␮m), and quiescent regimes. However, because retention times are higher (20–60 min), this process is inefficient when treating voluminous effluents at high flow-rates and high loadings capacities (>7 m3/h m2). Table 2 Main studies reported on the oily water separation by flotation. The Jameson cell, column flotation with colloidal gas aphrons (CGA) (pre-reagentized gas bubbles) and conventional columns are Authors Flotation system now being utilized in solvent extraction plants [37]. Here the flota- Al-Shamrani et al. [9], Bennett [15], Bensadok DAF – dissolved air tion devices are used in the discharge aqueous streams from the et al. [16], Zouboulis and Avranas [17], flotation SX-EW (solvent extraction-electrowinning) plant to recover the Alkhatib and Thiem [18], Russo and Silveira [19], Belhateche [20] organic liquor lost by entrainment into the aqueous phase. Thus, Welz et al. [13], Bennett [15], Belhateche [20], IAF – induced air flotation flotation can reduce organic losses and reduce potential environ- Angelidou et al. [21], Burkhardt et al. [22], mental problems. Strickland [23], Burkhardt [24], Medrzycka A modified jet flotation cell (MJC) has been designed in our lab to and Zwierzykowski [25,26], Medrzycka account for a better oil droplet capture by bubbles, for the decrease [27–30], Santander [31] Watcharasing et al. [11], Bennett [15], Nozzle flotation in the amount of “short circuit” observed in the conventional unit, Santander [31], Gopalratnam et al. [32] all at high loading treatment capacity, the latter being extremely Miller and Hupka [33], Beeby and Nicol [34], Centrifugal cyclone important in off-shore platforms [47]. This work presents results of Lelinski [35], da Rosa and Rubio [36] oil (petroleum)/water separation at laboratory (continuous work) Santander [31], Readett and Clayton [37] Jameson cell Xiao-bing et al. [10], Gebhardt et al. [38], Columns and DAF-columns and pilot plant, comparing process efficiencies between the MJC Wyslouzil [39] with the CJC (the conventional jet cell unit) in a small continuous Bande et al. [12] Electroflotation laboratory plant and operating, with the MJC, in a Brazilian offshore Watcharasing et al. [11] Flotation with gas aphrons platform. M. Santander et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244 239

Fig. 1. (a) Conventional jet ﬂotation cell, CJC; and (b) modiﬁed jet cell, MJC.

2. Experimental 2.1.2. Flotation studies The separation of oil from oil/water emulsions were conducted 2.1. Bench studies in the CJC and results compared to those obtained in the MJC as depicted in Fig. 1(a) and (b), respectively. 2.1.1. Materials Both lab flotation cells had 1.3 m3/h capacities and consisted of 2.1.1.1. Petroleum emulsions. These were prepared to simulate off- a downcomer of the jet, a flotation tank and a level control system. shore petroleum effluents, in a salty medium (75 g/L NaCl) using a The downcomer had 0.05 m diameter, 2 m height, immersed 0.1 m heavy oil, crude petroleum. Main physicochemical characteristics inside the concentric collector. The nozzle (6 mm diameter) had an of the oil were: real density of 0.890 kg/m3 (at 30 ◦C); a viscosity of air inlet orifice of 5 mm diameter. The cell tank (PVC) was 0.25 m 0.042 Ns/m2, surface tension 34.4 mN/m. diameter, 0.71 m height (35 L capacity) and the froth launder, 0.4 m The oily waters (oil/water emulsions) were generated in a con- length × 0.4 m width × 0.2 m height. Further details are shown in tinuous high-pressure homogenizer system [31]. The oil/water Table 3. mixtures were pumped, with a helical pump of high pressure In both cells the “capture” stage (collision–adhesion) to form oil (25 kgf/cm2) working at 2 m3/h capacity, containing varying con- floc/bubble aggregates occurs in the downcomer which are sepa- centrations of oil and NaCl (0–75 g/L), through stainless steel rated off from the water in the tank (phase separation) [42]. The homogenizer plates with holes of different diameters (3–7 mm). oily water was conditioned in a static mixer, with the PVA floccu- The two-phase mixture is subjected to intense turbulence and shear lant and fed to the jet cell. The reduction of pressure at the inlet by the conversion of pressure to kinetic energy, thereby leading to induced air intake which results in dispersed small bubbles. These breakup of the dispersed phase into small oil droplets, producing are forced to descend with the liquid counter to their buoyancy in stable emulsions with different size distributions. Oily waters were the direction of the flotation tank. characterized with respect to size distribution of the oil droplets In the MJC unit, a CJC has been redesigned placing an inter- using an on line particle size analyzer (Malvern model System 3601) nal cylinder (blind-end) which receive the downcomer suspension and concentration of oil (petroleum) by UV spectroscopy. Samples allowing all particles to enter immediately in the separation zone by for characterization were collected with the aid of samplers located the top (Figs. 1 and 2). Also, a packed bed (crowder) has been placed in the tube carrying the oily water feeding the flotation cell. Sam- at the flotation cell separator tank to stabilize the froth and facili- ples were collected every hour and results are the mean of about 6 tate the oil/bubble aggregates rising up, without much turbulence aliquots per day. to break them apart.

2.1.1.2. Flocculant. A non-ionic flocculant polymer, polyvinyl alco- Table 3 Jet flotation cells: CJC or MJC main operating and design parameters. hol (PVA) to destabilize the oil-in-water emulsions. Flow rate 1.3 m3/h Retention time of downcomer 10.9 s Retention time in the flotation cell 96.9 s 2.1.1.3. Frothers. Sodium Dodecyl Sulfate (SDS) and Dowfroth 1012 Diameter of downcomer 50.0 mm (DF-1012), a methyl-polypropylene glycol, were employed to assist Height of downcomer 2.0 m both, the air entrainment in the downcomer and the stabilization Diameter nozzle constriction 6.0 mm of the froth for keeping the floated oily flocs. Flotation cell volume 0.035 m3 240 M. Santander et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244

Results are mean values of triplicates tests and the standard error of the mean was about ±5% for a 95% conﬁdence interval. Some further operating parameters and cells design characteristics are summarized in Table 3.

2.2. Pilot plant studies in an oil ﬁeld platform

Studies were performed in a maritime platform, named PNA- 1 (from Petrobras, Rio de Janeiro, Brazil), in the MJC with petroleum/water emulsions from two different points of collection (Fig. 3). The studied parameters were the type of aspirated gas (natural gas or air), concentration of oil and type and concentration of flocculant (PVA and Dismulgan, a cationic polyacrylamide), but only optimized conditions (long-time experiments) are presented. Dis- tribution size drops in emulsions was not measured, but are known to be similar to those studied at laboratory scale. Samples were collected every hour and results are the mean of about 6 aliquots per day. Results are mean values of triplicates tests and the standard Fig. 2. Modified jet flotation. Details of the cylinder (blind) at the bottom and packed error of the mean was about ±5% for a 95% confidence interval. bed (crowder) at the top. Bench studied allowed to design and conduct studies of MJC flotation of the oily water effluent from the degassing vessel. This Flotation studies as a function of droplets size (volumetric size, oily water was pumped at a 5.0 m3/h flow rate and fed to the MJC d(4,3)) and oil concentration, were conducted in the CJC with using an helical pump of 8.3 m3/h capacity. The flocculant solution 3 L/min air and 20 L/min oil feed flowing through an static mixer was injected into the helical pump discharge pipe endowed with a (as a flash flocculator), 3 mg/L PVA and 28 mg/L DSS [47]. Flotation static mixer (800 mm long and 25 mm in diameter). runs lasted 6 h and samples (100 mL), of the treated water, were The MJC downcomer was 0.1 m diameter, 2 m height and was collected (for oil analysis), hourly in aliquots taken within 2–3 min. immersed 0.7 m inside the concentric collector. The residence time

Fig. 3. Oil ﬁeld extraction and oily waters treatment systems. PNA-1-oil ﬁeld platform from Petrobras, Rio de Janeiro, Brazil. M. Santander et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244 241

100 100 400 100 90 90 80 80 320 80 70 70

60 60 240 60 R (%) R (%) (mg/L) (mg/L)

50 50 f 40 40 160 40 [Oil] [Oil] 30 30 20 20 80 20 10 10 0 0 0 0 10 20 30 40 50 100 1000

d(4,3) (mm) [Oil]i (mg/L)

Fig. 4. Flotation of emulsified oil (residual oil) in the CJC as a function of oil droplets Fig. 6. Flotation of emulsified oil in the CJC. Effect of feed (initial) oil concentration size (volumetric diameter). [Oil]initial = 90–120 mg/L, [PVA] = 3 mg/L, [DSS] = 27 mg/L, ◦ on oil/water separation efficiency. [PVA] = 3 mg/L, [DSS] = 27 mg/L, 3 L/min air flow 3 L/min air flow rate, temperature 27–35 C and pH 6.5. rate, temperature 27–35 ◦C, d(4,3) = 23 ␮m and pH 6.5. of the slurry was 6.8 s for an estimated 40% holdup. The nozzle (12.25 mm diameter) had an air inlet orifice of 5 mm diameter. high projected area which facilitates collisions with small droplets The cell tank was 0.5 m diameter, 1 m height (196 L capacity and together with a suitable hydrodynamics. Yet, Pal and Masliyah [49] 2.36 min residence time) and the froth launder, 0.74 m diameter found, in studies of removal of chemically emulsified oils (using and 0.15–0.25 m deep. The blind bottom had 0.3 m diameter and non-ionic surfactants), that a decrease in the size of the bubbles 0.75 m height. All pieces were built using 316 stainless steel. decreased the efficiency of separation under strong stirring (hence the degree of mixing) within the collection zone of the flotation 3. Results and discussion column. An increase in the degree of mixing causes a reduction in the 3.1. Laboratory studies capture efficiency despite the increase in the frequency of collisions (due to the bubbles fineness). It is known that oil drops must 3.1.1. Conventional jet cell – CJC displace the thin liquid film that surrounds droplets and air bubbles Fig. 4 shows that if d(4,3) decreases from 40 to 10 ␮m the and this does not occur if the time required for the film thinning is removal percentage decreases from about 90% to 70%, increasing longer than the time of contact; in this case (with strong stirring), the residual oil concentration from 10 to 30 mg/L. If d(4,3) decreases the time decreases with the increase in the degree of mixing. from 40 to 12 ␮m, the concentration of droplets <10 ␮m increases Fig. 6 shows that, up to approximately 400 mg/L, the removal from zero to 37% (see Fig. 5). Thus, according to expectations the remains approximately constant at 80%. For concentrations higher collection efficiency of droplets by bubbles decreases with decreas- than 400 mg/L, this percentage decreases dramatically down to 60% ing their size as in flotation of mineral particles. This is because they oil removal. These values were obtained under optimized condi- follow the streamlines around the bubble and not collide with the tions involved various parameters [31]. The optimal parameters bubble surface which in this CJC (the bubbles) may be as big as found and reported in Santander [31] were: flow-rate: 20 L/h oily 0.1–0.8 mm [40,41]. water conditioned in a static mixer with 3 mg/L PVA, 28 mg/L de Strickland [23] and Sato et al. [48] reported that oil recovery SDS and 3 L/min air flow-rate. enhances while oil droplets size increases and bubbles decreases. The decrease in the oil separation efficiency in the CJC appears Sato explains that this is due to at least, two effects, the first geomet- to be due to, at least, the following phenomena: (a) Low oil feed ric and a second thermodynamics. Accordingly, small bubbles have concentration; (b) increase of the proportion of droplets <10 ␮m generating the problems already discussed; and (c) hydrodynamic turbulence in the flotation device at concentration >400 mg/L. 100 20 Also, in this CJC the instability of flow in the separation tank generates turbulence because the jet enters downwards with a high speed, increasing the drag of the finest oil droplets, especially those 80 Partial frequency (%) 15 either not flocculated or weakly attached to the air bubbles. d(4,3) = 12 mm Bubbles play the vital role of actually separating the solids (or 60 d(4,3) = 40 mm droplets in emulsions) from the liquid phase which ultimately leads 10 to their removal. It has been well demonstrated that the bubble size is one of the most important physical variables determining 40 flotation efficiency [50–54]. Bennett et al. [55] were the first to report the effect of bubble size and found that smaller bubbles were 5

Cumulative frequency (%) 20 more efficient in the flotation of coal. As Ahmed and Jameson [52] and Yoon [53] suggested, the use of small bubbles (in the range of 100–400 ␮m), and a more quiescent environment than the agi- 0 0 1 10 100 tated cells, it will improved the recovery rates associated with fine particles. Droplet diameter (µm) To date there is agreement that a decrease in the bubble size

Fig. 5. Comparison of droplet size distribution in the oily waters with different will improve oil droplet recovery because of increased surface area volumetric mean diameters. and, therefore, greater probability of contact between bubbles and 242 M. Santander et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244

Table 4 400 100 Effect of the average diameter of oil droplets on the MJC oil/water separation efficiency. Air flow rate, 5–9 L/min (maximum self-aspirated air). 320 80 d(4,3) (␮m) [Oil]initial (mg/L) [Oil]final (mg/L) R (%) 23.96 98.1 16.9 83

11.77 91.5 16.0 82 240 60 R (%) 24.43 161.7 25.6 84 CJC (mg/L)

12.15 161.2 21.8 86 f MJC 160 40 [Oil] particles. Nevertheless, if the bubbles are too small of course, they 80 20 will not be able to provide sufficient buoyancy to lift large particles, but this is not usually a limitation observed with oil droplets. For the same overall air flow rate, the number of bubbles in the suspen- 0 0 sion, and hence the surface area available for contact with particles, 100 1000 increases as the bubble size decreases. The bubble size obviously [Oil]i (mg/L) affects the bubble surface area flux and, hence, the carrying capacity of the device. It has also an effect on the capture efficiency and, Fig. 7. Comparison of oil/water flotation separation efficiencies between MJC and 3 consequently, the kinetic rate constant, which is often dependent CJC, as a function of the oil initial concentration. Flotation feed = 1.3 m /h, air flow rate ranging from 4 to 9 L/min (maximum self-aspirated), in MJC and 3 L/min in the on the ratio of the particle diameter to the bubble diameter, and to CJC. the bubble diameter itself.

Gas holdup (which controls the bubble surface area flux) in the Table 5 downcomer may reach as high as 50–70%, depending on the design Studies of removal of oil from oily water (effluent from the degassing vessel) by and operation conditions [44]. Many design and operating vari- flotation in the MJC. Feed rate, 5 m3/h. ables affect gas holdup [44,56]. In recent studies involving the CJC Flocculant flow rate, mL/min 100 or Jameson cell [44], the effect of several parameters, such as the Concentration of PVA, mg/L 3 nozzle diameter, the downcomer diameter, the free jet length, the Concentration of Dismulgan, mg/L 10 2 jet velocity and the ratio of air–feed flow rate on holdup and air Pressure, power input to the jet cell, kgf/cm 0.45 [Oil] Feed, mg/L 68.7 entrainment rate were reported. Holdup increased with increas- [Oil] Discharge, mg/L 29.7 ing downcomer diameter, jet velocity and length and air/feed flow Separation efficiency, % 56.8 rate ratio but with decreasing nozzle diameter. Higher holdup values were obtained if a small size nozzle diameter is used with large size downcomer diameter. re-flotation may occur inside the blind tube. This avoid or decreases the observed short-circuit in the conventional cell. Further, the fact that the discharge of the jet is performed in this cylinder bearing 3.1.2. Modified jet flotation cell (MJC) a packed bed, stabilize the flow and the froth allowing a minimum The test conditions were equal to those used in the CJC, except floc breakage. for the self-aspirated air flow which was found to be different. Table 4 shows that, with average oil droplets of the order 12 ␮m (in 3.2. Flotation studies with the MJC in an offshore platform diameter), was possible to reach removal efficiencies higher than 82%. This table also shows that removal efficiency is higher for the The studies were performed with samples collected in two dif- highest oil initial concentration. Removal (separation) efficiencies ferent points (see Fig. 3). of about 82% were attained with 90 mg/L (initial) oil concentrations and 86% with 160 mg/L. Results might be explained in terms of the 3.2.1. Oil/water separation in the effluent from the degassing degree of turbulence in the downcomer, which is a function of the vessel amount of air aspirated and appears to depend on the concentration Table 5 and Fig. 8 show the results of studies of oil removal using of oil. Here (in separated tests), it was found that for 600 mg/L of oily water from the degassing vessel (see Fig. 3). Results show that oil, the maximum airflow aspirated was 5 L/min and with 100 mg/L of oil, this increases to 10 L/min. Again, the higher degree of turbulence will make that the jet enters downwards with a higher speed, increasing the drag of the finest oil droplets, especially those either not flocculated or weakly attached to the air bubbles, decreasing the overall oil removal. Fig. 7 compares the separation performance of CJC and MJC, as a function of the initial concentration of oil, between 100 and 600 mg/L. The increase in the efficiency of separation of the oil droplets with respect to the conventional jet cell, with smaller sizes, again confirms a better capture of the oil flocs together with an efficient oily flocs/water separation at the flotation tank. Comparatively, the short circuit (oil droplets reported to the treated water, in this case) commonly observed in this type of jet cell is highly reduced by the concentric blind cylinder receiving the downcomer flux [31,57]. Thus, under the same experimental conditions, the modified cell presented (always) removal efficiencies, at least, 5% higher than the conventional. The concentric cilindrical Fig. 8. Separation of emulsified oil in water (degassing vessel) by flotation in the collector, in the MJC, avoid the carry-over of droplets of oil not fully MJC (loading capacity of 24.7 m/h). Flotation feed, 5 m3/h, 10 mg/L of Dismulgan adhered to the air bubbles. Because loaded bubbles ascend, a new and 3 mg/L of PVA. M. Santander et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 237–244 243

Table 6 4. Conclusions Results of the separation of oil from the oily water, effluent of the separator production, by flotation in the MJC. 10 mg/L of Dismulgan and 3 mg/L of PVA. Feed rate, 5m3/h. The removal of oil (petroleum) from oily water at concentrations ranging from 50 to 600 mg/L and volume mean diameter of Flocculant flow rate, mL/min 100 oil droplets in the order of 20 ␮m is efficient with both types of Concentration of PVA, mg/L 3 Concentration of Dismulgan, mg/L 10 jet flotation cell, the conventional CJC, or the modified, MJC. In Pressure, power input to the jet cell, kgf/cm2 1.15 this new design, the slurry enters directly (through a concentric [Oil] Feed, mg/L 123 blind-end tube) into the frothy phase after the capture of the oily [Oil] Discharge, mg/L 30 flocs by the bubbles. Because loaded bubbles ascend, a new re- Separation efficiency, % 76 flotation may occur inside the blind tube. This avoid or decreases the observed short-circuit in the conventional cell. Also, a crowder (packed bed) was included, at the top of the concentric blind-end tube, to stabilize the flow and the floated oily flocs. Results show the removal efficiencies were smaller than those obtained with syn- that this cell is more accurate than the conventional cell yielding thetic oily waters under similar conditions, namely temperature, high oil removal values and treated water with low oil levels. The salinity and concentration of oil. It is believed that this low effi- MCJ design resulted in an increase in the oil removal efficiency by a ciency was due to the presence of (a) residual chemicals such as steady 5% percentage points (from 80% in the CJC to 85% in the MJC) demulsifiers, and corrosion inhibitors that reduce the action of the in long operations. This increase in separation efficiency is due to PVA flocculant or (b) colloidal solids, coated with oil, which are not the decrease in drag of small droplets do not adhere to air bubbles as being flocculated by PVA. It was visually observed a high proportion the new mode of operation downcomer; hydrodynamic conditions of treated oily water with dispersed colloidal solids (even a sludge), of high turbulence, promotes flocculation (coalescence) of oil. The especially with the flocculant Dismulgan. testing of the MJC (5 m3/h) in a maritime platform demonstrated the efficiency of this rapid flotation device, obtaining high values of oil removal at a high throughput rate, reaching the allowable levels 3.2.2. Oil/water separation in the effluent from the production for discharge of oil in one single step. These values ranged between separator 20 and 30 mg/L of oil, with removal efficiencies of around 81%, at In Table 6 and Fig. 9 are described the operating parameters and 24.7 m3/h m2 throughput. The MJC shows simplicity in design and the results of oil flotation in the MJC with the oily water from the has great potential for the treatment of oily wastewaters at high production separator. In these tests the MJC operated with auto- rates. aspirated natural gas from the degassing tank. Results showed that removal efficiency is higher than those obtained with oily waters from the degassing vessel. This removal efficiency is probably due Acknowledgements to the fact that in this case, the oil was less emulsified (larger droplet size) and that the airflow aspirated was lower (less tur- The authors are grateful to FINEP, CNPq, CAPES, UFRGS and bulence inside the downcomer) and also no formation of sludge PETROBRAS (all Brazilian Agencies) for their financial support. Spe- was observed. The results obtained are within Brazilian emission cial thanks to O. de Aquino (Petrobras), S. Amaral, J.J. Rosa (UFRGS) standards of oil discharged to the sea (20–30 mg/L). and B. Zazzali (UDA-Chile), for their technical assistance. 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