Vacuum Evaporation of Waste Oil-in-Water Emulsions from a Copper

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Ind. Eng. Chem. Res. 2009, 48, 2100–2106

Vacuum Evaporation of Waste Oil-in-Water Emulsions from a Copper Metalworking Industry Gemma Gutie´rrez,† Jose´ M. Benito,‡ Jose´ Coca,† and Carmen Pazos*,† Department of Chemical and EnVironmental Engineering, UniVersity of OViedo, C/ Julia´n ClaVerı´a 8, 33006 OViedo, Spain, and Department of Chemical Engineering, UniVersity of Burgos, Plaza Misael Ban˜uelos s/n, 09001 Burgos, Spain

The performance of vacuum evaporation was evaluated for the treatment of waste oil-in-water (O/W) emulsions, one synthetic (Multiroll) and two semisynthetic (Unopol and Divinol), used in a copper metalworking industry. A destabilization/settling pretreatment step using Nalco 71243 (NAL, a quaternary polyamine) as flocculant and CaCl2 and AlCl3 · 6H2O as coagulants was also investigated. The quality of the final aqueous effluent was evaluated in terms of chemical oxygen demand (COD), turbidity, conductivity, and biological oxygen demand (BOD5). COD reduction greater than 99% was achieved. The pretreatment step did not significantly improve either the evaporation rate or the quality of the condensate. Moreover, evaporation of a totally exhausted Unopol waste emulsion carried out in an industrial falling film evaporator showed good agreement with results obtained at the laboratory scale. 1. Introduction The metalworking industry utilizes large amounts of oils or oil-in-water (O/W) emulsions for lubrication and cooling purposes. These emulsions lose their functional properties with aging. The oily waste must be collected and treated before disposal to remove the free, soluble, and emulsified oil. There are several methods available for separation of emulsions, such as sedimentation, centrifugation, deep bed filtration, ultrafiltration, and evaporation.1-4 The process efficiency depends on the nature of the emulsion, and a proper combination of the aforementioned methods generally yields the best results.5,6 The use of water-based synthetic and semisynthetic emulsions is increasing in industrial practice due to their low oil content. Evaporation is more effective than other processes for the treatment of these systems because of the low oil concentrations present in these emulsions. Their treatment by mechanical methods is difficult either because of the very small oil droplet size (semisynthetic emulsions) or no droplets being present (synthetic emulsions). In the traditional methods, the oily phase can be removed from the emulsion, but water-soluble organic compounds remain in the aqueous phase leading to high chemical oxygen demand (COD) values of the discharged effluent.3 Evaporation works well for oily wastewaters because the emulsion is firmly broken and the water can be reused as process water or to reformulate new emulsions. Although this technique is now used for the treatment of waste O/W emulsions in some existing plants, its performance has not been fully studied, and only scarce information can be found in the literature. Unfortunately, evaporation is energy intensive even though its energy consumption can be reduced by working at low pressures, i.e. as vacuum evaporation. The energy consumption for the industrial treatment of 1 m3 of a waste O/W emulsion has been calculated7 for three techniques: it was 70 kWh for a vacuum evaporation treatment, whereas it was only 18 and 5 kWh for ultrafiltration or centrifugation treatment, respectively. Neverthe* To whom correspondence should be addressed. E-mail: cpazos@ uniovi.es. † University of Oviedo. ‡ University of Burgos.

less, evaporation is attractive because the quality of the aqueous effluent obtained is much higher for vacuum evaporation than for the other treatments. Moreover, the oil residue recovered from vacuum evaporation has low water content and may be burned for energy production. A pretreatment step is often considered in the treatment of mineral emulsions6,7 since much of the free and tramp oils present in the waste emulsion can be rather readily removed. In the case of evaporation, pretreatment should not only improve the quality of the final aqueous effluent, but it would also increase the time between evaporator cleaning cycles, since a lower load of water-insoluble components will be fed to the evaporator, minimizing fouling of the heat exchange tubes. However, these advantages are not so clear for the treatment of synthetic and semisynthetic emulsions since they carry lower oil content than do mineral emulsions. That fact would be studied in the present work. Factors such as the heat transfer rate8-10 and operating pressure2,11 play an important role in evaporator performance. The main drawback of evaporation is the aforementioned high energy requirement, although it is more acceptable under vacuum conditions. Moreover, compounds with a vapor pressure similar to that of water, such as some demolding agents used to remove metallic material from a mold in the casting process,1 may appear in the waste and pollute the aqueous evaporator condensate. Another source of condensate contamination might be organics formed by thermal degradation of compounds present in the original O/W emulsion formulation, i.e., nonionic surfactants degraded by destruction of their ethoxylated bonds at 400-500 °C by contact with the metal surfaces in rolling processes. The composition of the emulsion, the type of emulsifiers (mainly surfactants), and their concentrations influence the evaporation process. High concentrations may decrease the evaporation rate, especially for emulsifiers containing a long hydrophobic chain.12-15 Furthermore, foam formation induced by emulsifiers will provide more air/water interfacial area, enhancing the water evaporation rate.16 Foaming may result, however, in some organic entrainment into the vapor and condensate. Electrostatic repulsion or steric barriers between oil droplets can affect the evaporation process: oil droplets

10.1021/ie801054d CCC: $40.75  2009 American Chemical Society Published on Web 01/14/2009

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Figure 1. Experimental apparatus for vacuum evaporation. Table 1. Characteristics of the Waste O/W Emulsions Studied emulsion

COD BOD5 turbidity conductivity (mg/L) (mg/L) (NTU) (µS/cm) pH

Multiroll (8 months used) 29600 Unopol (3 months used) 304800 Divinol (1 year used) 158700

315 155 188

1380 29300 58500

5750 4820 1673

9.0 8.5 8.3

remain separated from the vapor phase by a water film, and the oil must diffuse across this water film to reach the vapor phase. For the same oil, nonionic surfactants enhance oil evaporation more than ionic surfactants, thus lowering the quality of the final aqueous effluent. Electrostatic repulsion between oil droplets in ionic surfactant-stabilized emulsions thicken the water film relative to those emulsions stabilized by nonionic surfactants.17,18 This paper reports laboratory studies of vacuum evaporation for the treatment of one synthetic and two semisynthetic waste O/W emulsions based on commercial formulations from a copper metalworking industry to examine the recovery of water and oil residues. The effect of a destabilization/settling pretreatment step on evaporation performance is also examined. Finally, laboratory-scale evaporation results were compared with those obtained in an industrial falling film evaporator for the treatment of a totally exhausted O/W emulsion.

Figure 2. Effect of ∆TBE on the COD of aqueous effluent obtained after vacuum evaporation at 10 kPa for three waste O/W emulsions: Multiroll (A), Unopol (B), and Divinol (C).

Figure 3. Effect of ∆TBE on evaporation rates for waste O/W emulsions at 10 kPa.

2. Materials and Methods Evaporation experiments were carried out using a Bu¨chi R205 evaporator (Figure 1). This consisted of a rotating flask immersed in a heating bath, in which a weighted sample of emulsion was heated to the desired temperature. The vapor was condensed in a water-condenser and collected in a receiving flask. The process was carried out at low pressures by means of a vacuum pump provided with a pressure control device. The vapor temperature was recorded continuously. The rotor speed of the feed flask was set at 60 rpm in order to ensure good mixing. Waste oil-in-water emulsions were provided by a local northern Spain copper manufacturing company. This continuouscasting plant uses several processes. First, copper cathodes with a copper purity higher than 99.9% are loaded into a shaft furnace and melted by natural gas burners. The liquid copper is fed

Figure 4. Weight percentages of oily residue remaining after evaporation of the three O/W emulsions at 10 kPa and different ∆TBE.

between a water-cooled hollow casting wheel and belt and is formed into a continuous 80 mm × 60 mm rod. This rod passes through a rolling mill which progressively reduces its diameter

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Figure 7. Weight percentages of oily residue remaining after evaporation of the three O/W emulsions at ∆TBE ) 105 °C and different operating pressures.

Figure 5. Effect of operating pressure on COD of aqueous effluent after evaporation at a ∆TBE ) 105 °C for three waste O/W emulsions: Multiroll (A), Unopol (B), and Divinol (C).

Figure 6. Effect of operating pressure on evaporation rates for waste O/W emulsions at ∆TBE ) 105 °C.

to an 8 mm copper wire rod. The rod then undergoes a pickling treatment with isopropyl alcohol to prevent coil oxidation. Afterward, the 8 mm wire rod undergoes a smoothing process to produce 2 mm wire, which is subsequently processed in a wire drawing machine to obtain copper wire of 0.25 mm diameter. The resulting wire is processed in an annealing furnace to improve product quality. Waste O/W emulsions used in this study were generated in different parts of the process: (i) A synthetic emulsion used in the rolling process (Multiroll) made as a 2% v/v commercial concentrate in water; commercial name: Multiroll Cu (Zeller + Gmelin GmbH & Co. KG, Eislingen, Germany). (ii) A semisynthetic emulsion used for lubrication and cooling in the smoothing process (Unopol), 13-14% v/v commercial mineral oil concentrate in water; commercial name: Unopol G 560 (Carl Bechem GmbH, Hagen, Germany).

Figure 8. Evaporation rate (A) and COD values of condensates (B) after evaporation of emulsions at 10 kPa and ∆TBE ) 54 °C. Data correspond to samples without pretreatment (filled symbols) and with destabilization/ settling pretreatment (open symbols) before evaporation.

(iii) A semisynthetic emulsion, used in the wire drawing process (Divinol), a 5% v/v commercial concentrate, based on naphthenic hydrocarbons containing both anionic and nonionic surfactants, in water; commercial name: Multidraw Cu MF (Zeller + Gmelin GmbH & Co. KG, Eislingen, Germany). Table 1 shows some characteristics of the waste O/W emulsions employed in this work. These three emulsions had been used in the plant for 8 months, 3 months, and 1 year, respectively. Evaporation experiments were performed with 100 g of waste O/W emulsion sample. Evaporation rate (E), expressed as the mass of sample evaporated (i.e., condensate, mainly water) per unit time, was measured for each experiment. The condensate was collected in the receiving flask until the measured vapor temperature suddenly decreased, which indicated the depletion of water in the flask and the end of the experiment. Then, it

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Figure 9. Visual aspect of Divinol emulsion samples after destabilization (0.05 M AlCl3) and settling and pH adjustment, before evaporation.

Figure 12. Turbidity, pH, and conductivity of condensates after destabilization (0.05 M AlCl3) and settling pretreatment and subsequent vacuum evaporation of Divinol emulsions at different pH. Figure 10. Particle size distribution (volume-weighted mean diameter) of Divinol emulsion samples at different pH after destabilization (0.05 M AlCl3) and settling and before evaporation.

Figure 11. Effect of pH on the quality of the evaporation condensate of the Divinol emulsion after pretreatment (P ) 10 kPa, ∆TBE ) 54 °C).

was weighed using an electronic scale (Kern PRJ 1200-3N), with a precision of (0.001 g. The time needed to collect the condensate was measured with a stopwatch, and it varied from 8 to 25 min, depending on operation conditions. The measured error in the evaporation rate values presented in this work is less than 2%. Qualities of the feed and the aqueous product (condensates) were evaluated in terms of chemical oxygen demand (COD), turbidity, conductivity, pH, and biological oxygen demand (BOD5). The COD analysis was carried out by the reactor digestion method19 using a Hach DR2010 UV spectrophotometer. Turbidity was measured using a Hach ratio XR turbidimeter. The conductivity meter was a Crison Micro CM 2202. BOD5 was determined with a WTW Oxitop IS6. The pH measurements were performed with a Crison pH 25 pH-meter. The errors of the analysis methods were on the order of (1 mg/L for COD and BOD5, (1 µS/cm for conductivity, (0.1 units for pH, and (0.1 NTU (from 0 to 100 NTU) and (1 NTU (>100 NTU) for turbidity measurements.

Figure 13. Properties of the evaporation condensate after treatment of Unopol waste emulsion with industrial and laboratory-scale evaporators.

The selection of the flocculants/coagulants and their concentration was guided by centrifugation tests in order to evaluate the efficiency of the destabilization/settling pretreatment step for oil removal. Centrifugation was carried out in a Wifug Labor 50 batch centrifuge at 3000 rpm (475g) for 15 min. Nalco 71243 (NAL, a quaternary polyamine supplied by Nalco Europe (Leiden, The Netherlands)) and CaCl2 and AlCl3 · 6H2O (Panreac S.L., Barcelona, Spain) were used as flocculant and coagulant salts, respectively, to break the emulsion. Particle size distributions were measured following the laser light scattering technique and using a Mastersizer S long bench apparatus (Malvern Instruments Ltd., Malvern, UK). Samples were first diluted with deionized water in order to prevent multiple scattering effects in the measurements, and then, they were circulated through the measuring zone using a Hydro SM small volume sample dispersion unit. The dilution did not affect the results as no significant changes in particle size distributions were observed in diluted samples ranging from 1:10 to 1:100 dilution ratios. Five replicate measurements were conducted for

2104 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 Table 2. Properties of Aqueous Phases Obtained after Destabilization and Settling Treatments of the Waste O/W Emulsions emulsion

demulsifier

COD (mg/L)

turbidity (NTU)

conductivity (µS/cm)

pH

Multiroll (8 months used) Unopol (3 months used) Divinol (1 year used)

500 mg/L Nalco 71243 0.2 M CaCl2 0.05 M AlCl3

11150 34450 8575

266 19.7 1580

2070 23700 2010

3.8 7.9 3.6

Table 3. Initial Properties of Waste Unopol Emulsion used for Industrial-Scale Evaporation emulsion

COD (mg/L)

BOD5 (mg/L)

turbidity (NTU)

conductivity (µS/cm)

pH

Unopol (4 years used)

11600

102

7650

5550

9.1

each sample at room temperature. The particle size distribution was calculated with the built-in Mastersizer software, and it was based on the best fit between the experimental measurements and the Mie theory. The intrinsic size distribution derived by this technique is volume based. Thus, particle size results are reported as the volume-weighted mean diameter d4,3 ) Σnidi4/ Σnidi3, where ni is the number of particles with diameter di. 3. Results and Discussion 3.1. Effect of Thermal Driving Force and Heating Rate. The rate of heat transfer is the key parameter in evaporation, since it influences bubble size,9 bubble life, and the manner in which bubbles break.8 An increase in the temperature of the heating medium enhances the evaporation rate (E), but decreases the quality of the condensate.20 Previous studies have shown that a very hot surface could have a negative effect on the quality of the condensate, since a vapor layer is formed between the emulsion and the surface. This decreases the rate of heat transfer to the emulsion.10 This situation was prevented by rotating the feed flask to mix the emulsion and thus maintain more uniform heat transfer. Experiments were carried out at constant pressure but at several bath temperatures. First, the operating pressure was fixed at 10 kPa for trials at bath temperatures of 100, 150, and 180 °C. These correspond to temperature differences between the bath and the boiling water of 54, 104, and 134 °C, respectively. The effects of the difference between the bath temperature and the evaporation temperature (∆TBE) on the final aqueous effluent quality, evaporation rate, and the final percentage of oily residue after evaporation (i.e., the fraction of the feed remaining with respect to the initial feed mass) are shown for the three emulsions tested in Figures 2, 3, and 4, respectively. Figure 3 shows that the evaporation rate increases with increasing ∆TBE. Similar results were observed in previous studies.20 However, the COD of the condensate became higher as ∆TBE increased, especially for the semisynthetic emulsions (Figure 2). These results may be attributed to the evaporation of the more volatile components of the oil. Moreover, a phase inversion of the emulsion from O/W to W/O might occur when more than 80-85% of the waste emulsion is evaporated and facilitate the evaporation of the volatile organic components or even allow entrainment of some oil into the vapor phase, which would increase the COD of the condensate. That trend is clearer for higher ∆TBE, probably because the boiling is more violent and because the phase inversion point of an emulsion depends on temperature. The synthetic emulsion (Multiroll) yielded the lowest amount of final oily residue (Figure 4) because of the negligible oil content in its original formulation. In all cases, a slight decrease in residual oil was observed as ∆TBE increased. 3.2. Effect of Operating Pressure. The effect of the operating pressure was studied at a constant ∆TBE value of 105 °C. Operating pressure was investigated in previous work but

not at constant ∆TBE.2 Pressures of 10, 25, and 40 kPa were used at bath temperatures of 100, 150, and 180 °C, respectively. The COD of the aqueous effluent, evaporation rate, and the final portion of oily residue after evaporation are shown in Figures 5-7. Increases in both the evaporation rate and the condensate COD values were observed as the operating pressure was increased. These increases were minor, however, and not as large as those observed for evaporation at different ∆TBE values. No significant changes in the final volume of oily residue were observed with increased operating pressure. Once again, as might be expected, the Unopol emulsion produced the largest volume of concentrated oily residue because of the larger initial amount of commercial concentrate in the emulsion. Nevertheless, the COD values of the condensate for this emulsion are similar to those for the other fluids. 3.3. Pretreatment of the Waste Emulsions. A destabilization/centrifugation pretreatment step to remove part of the initial oil content should lead to reduced frequency of evaporator cleaning and lower energy costs.6 Such a pretreatment led to good results in the treatment of mineral oil-in-water emulsions.2 The three emulsions studied here were centrifuged at 3000 rpm (475g) for 15 min using different flocculant and coagulant salts to determine the critical coagulation concentration (CCC). Best results were obtained using 500 mg/L of Nalco 71243 flocculant for the synthetic emulsion (Multiroll) and 0.2 M CaCl2 and 0.05 M AlCl3 for the semisynthetic emulsions Unopol and Divinol, respectively. Subsequently, 200 mL emulsion samples were allowed to settle for 16 h after adding the optimum concentration of flocculant and coagulant salts. Properties of the resulting aqueous phases are shown in Table 2. A clear decrease of the evaporation rate was observed after emulsion pretreatment, and it also led to higher COD values of the effluent (Figure 8). These results could be explained by the change in pH that occurs when demulsifiers are added (Table 2): pH influences the zeta potential of emulsions and, therefore, the emulsion stability.21 Most of oil droplets being negatively charged, a decrease in pH lowers the net charge of the electric double layer, reducing the repulsive forces between droplets and enhancing coalescence.13 This pH decrease also allows the formation of acid species in the waste emulsion that may evaporate and decrease the quality of the condensate. Coagulant salt addition decreases the pH of the emulsion and causes the formation of Ca(OH)2 and Al(OH)3 precipitates.22,23 The presence of salts reduced the emulsion evaporation rate and increased the boiling point.24 However, the evaporation rate decreases when the oil droplet size increases.25,26 The effect of pH on the evaporation process was studied using samples of the semisynthetic Divinol emulsion. The pH of several samples was adjusted with 1 M NaOH after the emulsion was destabilized and settled by addition of 0.05 M AlCl3. Pictures of the Divinol samples are shown in Figure 9. A white precipitate was observed at pH values between 5 and 10, mainly Al(OH)3. The precipitate turned blue at a pH between 10 and 11. This is characteristic of Cu(OH)2. At higher pH, the precipitate disappeared, and the solution became dark-blue, likely because of the formation of soluble compounds of

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[Cun(OH)2n-2] . The presence of these precipitates is demonstrated by particle size measurements (Figure 10). Particle size, reported as the volume-weighted mean diameter, increases as pH increases, indicating the presence of the precipitates. A reduction of particle size at pH values higher than 12.4 was observed, due to the formation of the soluble complexes and dissolution of the precipitates. Particle size distribution corresponding to pH values of 5.2 and 10.2 are similar to the one obtained at pH 7.2, although it is difficult to see the similarities in Figure 10. Divinol samples, prepared at different pH values, were evaporated, and the quality of the aqueous effluent was analyzed. As shown in Figure 11, COD decreased as the pH of the emulsion was increased by adding NaOH after pretreatment. Conductivity, pH, and turbidity of the condensates at different pH values are shown in Figure 12. Condensate conductivity is strongly reduced by the presence of the first precipitates. It increases again at high pH values due to precipitate dissolution. pH values appear to remain constant for all the samples, whereas a slight decrease in turbidity is observed when precipitates are present in the samples to be evaporated. 3.4. Evaporation Process in an Industrial Unit. Evaporation of the Unopol emulsion was carried out in a Prowadest 60/1 industrial falling film evaporator (Condorchem Ibe´rica S.L., Barcelona, Spain) with forced circulation and steam compression for a condensate output of 60 L/h, for comparison with laboratory scale results. The waste emulsion employed in this study had been used by the company for 4 years, so it was at the end of its lifetime. Its properties, prior to the treatment, are shown in Table 3. It must be pointed out that this was a totally exhausted and destabilized O/W emulsion, whose original components had undergone severe thermal degradation. It was stored in a settling tank after use, where it was mixed with alkaline wash waters from the copper rolling process. This emulsion was fed to the falling film evaporator from the bottom of the settling tank in order to avoid all the free and hydraulic oils collected at the surface. The history of this emulsion explains the differences observed when its properties are compared with the Unopol waste emulsion studied in the previous laboratory tests. The latter had been used for only 3 months (Table 1). A sample of the exhausted waste emulsion fed to the industrial evaporator was also collected for testing in the laboratory rotary vacuum evaporator. The operating pressure for both cases was 60 kPa, similar to that employed in the previous laboratory experiments. As shown in Figure 13, no significant differences were observed between the two condensate products obtained at laboratory and industrial scales. Low pH and high conductivity suggest small amounts of acid and salts were present in the industrial evaporator, probably because of the difficulty of removing scale fouling products from the evaporator tubes during cleaning. The higher COD value of the condensate for the totally exhausted Unopol O/W emulsion compared with that from the 3 month old emulsion shown in Figure 8B is mainly due to the severe thermal degradation of its original components. During use, these can be degraded and transformed into more volatile compounds with boiling points close to that of water or to compounds that may form azeotropic mixtures with water and end up in the vapor condensate. 4. Conclusions Evaporation is a suitable technique for treatment of synthetic and semisynthetic waste O/W emulsions, resulting in a 99% COD reduction. Operating conditions play an important role in

determining process efficiency since an increase in the ∆TBE increases the evaporation rate but decrease the quality of the condensate. An increase in the operating pressure exhibits similar but lesser effects. The use of a destabilization/settling pretreatment step did not significantly improve either the evaporation rate or the quality of the condensate in the treatment of waste synthetic and semisynthetic O/W emulsions as it does for mineral emulsions. In all cases, a proper selection of the destabilization agent and pH are needed because reactions with ions present in the emulsion may result in negative effects on the evaporation process. Vacuum evaporation of a totally exhausted O/W emulsion, carried out in an industrial falling film evaporator, showed good agreement with results obtained at the laboratory scale. From the experimental results, it can be concluded that it would be advantageous to periodically withdraw a known volume of an O/W emulsion from an industrial process and to replace it with a similar volume of fresh emulsion to improve emulsion lifetime. The purged O/W emulsion waste can then be treated by vacuum evaporation to reuse the vapor condensate as process water or for emulsion reformulation. Vacuum evaporation may be used for waste treatment even if the O/W emulsion is totally exhausted, but in this case a lower quality vapor condensate will be obtained. Acknowledgment One of the authors (G.G.) was the recipient of predoctoral fellowship from FPI Program (MEC, Spain). Financial support by the Ministerio de Educacio´n y Ciencia (MEC, Spain) and the European Commission (project ref.: CTQ2004-01413, European Regional Development Fund) is also gratefully acknowledged. Literature Cited (1) Can˜izares, P.; Garcı´a-Go´mez, J.; Martı´nez, F.; Rodrigo, M. A. Evaluation of a simple batch distillation process for treating wastes from metalworking industries. J. Chem. Technol. Biotechnol. 2004, 79, 533– 539. (2) Gutie´rrez, G.; Cambiella, A.; Benito, J. M.; Pazos, C.; Coca, J. The effect of additives on the treatment of oil-in-water emulsions by vacuum evaporation. J. Hazard. Mater. 2007, 144, 649–654. (3) Lobo, A.; Cambiella, A.; Benito, J. M.; Pazos, C.; Coca, J. Effect of a previous coagulation stage on the ultrafiltration of a metalworking emulsion using ceramic membranes. Desalination 2006, 200, 330–332. (4) Lobo, A.; Cambiella, A.; Benito, J. M.; Pazos, C.; Coca, J. Ultrafiltration of oil-in-water emulsions with ceramic membranes: influence of pH and crossflow velocity. J. Membr. Sci. 2006, 278, 328–334. (5) Benito, J. M.; Rı´os, G.; Pazos, C.; Coca, J. Methods for the Separation of Emulsified Oil from Water: a State-of-the-Art Review. In Trends in Chemical Engineering, Research Trends; Trivandrum, India, 1998; Vol. 4, p 203-231. (6) Benito, J. M.; Rı´os, G.; Ortea, E.; Ferna´ndez, E.; Cambiella, A.; Pazos, C.; Coca, J. Design and construction of a modular pilot plant for the treatment of oil-containing wastewaters. Desalination 2002, 147, 5–10. (7) Gutie´rrez, G.; Lobo, A.; Allende, D.; Cambiella, A.; Pazos, C.; Coca, J.; Benito, J. M. Influence of coagulant salt addition on the treatment of oil-in-water emulsions by centrifugation, ultrafiltration, and vacuum evaporation. Sep. Sci. Technol. 2008, 43, 1884–1895. (8) Abu-Zaid, M. An experimental study of the evaporation characteristics of emulsified liquid droplets. Heat Mass Transf. 2004, 40, 737–741. (9) Hetsroni, G.; Mosyak, A.; Pogrebnyak, E.; Sher, I.; Segal, Z. Bubble growth in saturated pool boiling in water and surfactant solution. Int. J. Multiph. Flow 2006, 32, 159–182. (10) Januszkiewicz, K. R.; Riahi, A. R.; Barakat, S. High temperature tribological behaviour of lubricating emulsion. Wear 2004, 256, 1050–1061. (11) Di Palma, L.; Ferrantelli, P.; Merli, C.; Petrucci, E. Treatment of industrial landfill leachate by means of evaporation and reverse osmosis. Waste Manage. 2002, 22, 951–955.

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ReceiVed for reView July 9, 2008 ReVised manuscript receiVed November 26, 2008 Accepted December 3, 2008 IE801054D