Modified Resin Coalescer for Oil-in-Water Emulsion Treatment: Effect

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Modified Resin Coalescer for Oil-in-Water Emulsion Treatment: Effect of Operating Conditions on Oil Removal Performance Yan Bo Zhou, Li Chen, Xiao Meng Hu, and Jun Lu* College of Resource and EnVironmental Engineering, East China UniVersity of Science and Technology, No. 130 Meilong Road, Shanghai 200237, China

The application of modified resin as a filter medium treating the anionic surfactant stabilized oil-in-water emulsion was investigated. In this study, emulsion breaking was accelerated by the grafting cetyltrimethylammonium bromide on polystyrene resin, so the need for expensive specific chemicals was eliminated. The results indicated that, as a new type of coalescence material, the modified resin has incomparable advantages over polypropylene, ceramic, and activated carbon. A series of experiments were performed to evaluate the effect of flow velocity, bed height, oil concentration, temperature, and pH value on the oil removal performance of the modified resin coalescer. More than 80% of emulsified oil was removed under optimal operating conditions: flow velocity 60-180 mL/h, bed height 20-40 cm, temperature 20-60 °C, and pH value 2-10. The results further indicated that modified resin bed coalescer is feasible to be used in the treatment of oilin-water emulsion due to integration of both advantages of chemical demulsification and coalescence process. 1. Introduction Wastewaters contaminated with oil are produced in numerous industrial processes and activities such as the food industry, petrochemical industry, mechanical and metallurgical industries, etc. Significant quantities of oily wastewater are rejected into nature or ordinary sewage systems because of nonadaptation and toxicity for the biochemical treatment.1-3 Besides that, ecological considerations have led governments to promulgate environmental regulations on the effluent quality to prevent environmental pollution. The Chinese integrated wastewater discharge primary standard, for example, allows a maximum concentration of 5 mg/L mineral oil into groundwater.4 Therefore, meeting progressively more stringent discharge effluent quality requirements has been a major issue faced by any environmental engineer. Generally, the present type of oil in water based on size and stability can appear in three ways: free oil, unstable dispersed oil, and emulsified oil.5 In the first two cases, the oil that is simply intermixed with water is easy to deal with. However, the emulsion containing finely dispersed amounts of oil is difficult to treat due to the presence of surfactants and cosurfactants. Treating this kind of wastewater is often costly and time consuming because of the interfacial films and the electrostatic repulsion forces among fine droplets.6 Emulsion can be broken by many methods such as chemical, electrical, or physical means. Despite lots of effort expended in this field, there remains a need for improved methods and apparatus for treating emulsified oil wastewater. Currently, the common methods commercially available for emulsion breaking involve chemical de-emulsification followed by air flotation or gravity separation.7-9 But, these processes require the use of expensive specific chemicals and consequently suffer from problems such as extra volumes of sludge and corrosion. Other methods that are used for the treatment of oily water such as vacuum evaporation,10 filtration, and membrane processing 5,11,12 also have respective disadvantages including energy consumtion, operational complexity, susceptible to influent quality, etc., without acceptable efficiency. * Corresponding author. Tel.: +86 21 64252058. Fax: +86 21 64252737. E-mail: [email protected].

Recently, a certain number of studies have focused on coalescence, which is an effective process used to destabilize and remove finely dispersed particles from wastewaters. These studies have shown that a coalescer is effective in reducing high levels of oil and grease in appropriate particle size range in the synthetic and actual industrial effluents treatment 13-15 that achieve technically and economically efficient oil-in-water (O/ W) emulsion separation. The efficiency of oil removal from water largely depends on the properties of the filter medium,16 so the filter medium is the key point in coalescence process for oily wastewater treatment. However, the application of commonly used filter mediums such as activated carbon and polypropylene is limited due to high cost and inefficient regeneration,17 which needs further improvement and modification. In some cases, emulsion breakers are added to enhance and accelerate the separation process. Therefore, researchers have synthesized a series of cationic polyelectrolytes containing quaternary ammonium salts as demulsifiers via polymerization or polycondensation reactions to improve the efficiency of breaking O/W emulsions.7,18 A novel approach, which has both advantages of chemical demulsification and coalescence process such as accelerating emulsion breaking and preventing sludge producing, is to prepare a filter medium for coalescence by using polystyrene resin modified with quaternary ammonium. The results from our previous work19,20 indicated that modified resin is an advantageous coalescing media, exhibiting better separation efficiency than other materials in n-octane wastewater treatment. In this paper, a modified resin coalescer was developed for the treatment of emulsified diesel oil wastewater and compared with other packing materials, polypropylene granular (PP), granular activated carbon (GAC), and ceramic filter (CF), on oil removal performance. As a potential application, the effect of various operating parameters such as the flow velocity, bed height, influent oil concentration, pH, and temperature on oil separation efficiency of the modified resin coalescer was also studied in particular. 2. Materials and Methods The laboratory scale coalescence unit used in this study consists of an agitator feed tank, a column coalescer, a peristaltic

10.1021/ie8012242 CCC: $40.75  2009 American Chemical Society Published on Web 12/24/2008

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Figure 1. Schematic of the experimental apparatus: (1) agitator feed tank, (2) peristaltic pump, (3) flow meter, (4) column coalescer, (5) sedimentation tank.

pump, a flow meter and a sedimentation tank, as schematically shown in Figure 1. The column coalescer (450 mm in height and 20 mm in diameter) has a water bath cannula providing constant temperature during experiments. Two types of filter medium were investigated in this study: organic (PP and polystyrene resin) and inorganic (GAC and CF). PP used with specification of S700 was obtained from Sinopec Yangzi Petrochemical Co. Ltd.; polystyrene resin (JK006) that was purchased from Shanghai Huazhen Sci. & Tech. Co., Ltd., is a styrene cationic exchange resin with specification of Amberlit IRA-63. After it was fully washed in hot water and pretreated by acid and base to remove impurities, unmodified resin (R-H) was produced. Modified resin (R-CTAB) was produced from R-H modified by cetyltrimethylammonium bromide (CTAB), whose preparation process was discussed in a previous paper.19 CTAB was purchased from Shanghai Chemical Reagents Co., and its identification and basic properties are shown in Table 1. A coal-based GAC with mesh size of 8 × 30 (Calgon Carbon Corp.) and a spherical-shaped CF (Pingxiang Chemical Packing Group Corp.) with particle diameter of 1.0-2.0 mm were selected to compare with the organic filter medium. Diesel oil #0 (SINOPEC Shanghai Petrochemical Co. Ltd.) is a catalytic cracking and hydrofining distillate of vacuum residuum and deasphalted oil having a density of 843 kg/m3 (20 °C). Anionic surfactant, sodium dodecylbenzenesulfonate (SDBS) (Shanghai Chemical Reagents Co.), was used as the stabilizing agent and other chemicals (China National Medicines Co.) were analytical grade and used without further purification. A synthetic diesel oil emulsion with oil concentration of 1000 mg/L and mean droplets diameter of 10 µm was freshly prepared by mixing appropriate amount of SDBS, diesel oil, and tap water at 5000 rpm in a blender (JSF-400, Shanghai Weida) for 10 min. After dilution into particular concentrations, the synthetic emulsion wastewaters were introduced by peristaltic pump (D100B, Shanghai Huxi) into the column coalescer passing through a flow meter (LZB-3, Beijing Flowmeter Factory). The analysis of oil removal efficiency of the column coalescer was carried out under various operational conditions. After a desired coalescence interval, the free oil at the top of the Table 1. Identification and Properties of the Modifying Agent

Figure 2. Comparison of oil removal efficiency of different packing materials (flow velocity ) 100 mL/h; bed height ) 30 cm; oil concentration ) 500 mg/L; pH ) 7.0; 30 °C).

sedimentation tank was discharged intermittently to the oil recovery system and the aqueous samples were collected from the bottom for analysis. Oil concentrations were determined using UV spectrophotometer (UV-2100, Unico) to measure absorbency at 224 nm wavelength. The relative standard deviations (RSD) of this method for the standard solution (10.0 mg/L) were 1.5% and 2.8% for repeatability and reproducibility, respectively, and the average relative error was 0.2%. The zeta potentials of oil droplets were measured by a Zetasizer (Nano Series, Malvern, UK); average values were obtained from three replications. pH values were measured by a pH meter (Delta320, Mettler Toledo). 3. Results and Discussion 3.1. Comparison of Oil Removal Efficiency for Different Packing Materials. The comparison between RCTAB and other materials for oily wastewater treatment was the basic test for further study. The performance of all the materials was examined by oil removal with initial oil concentration of 500 mg/L and flow velocity of 100 mL/h. The oil removal for all investigated materials in the column coalescer is presented in Figure 2. The results indicate that oil removal efficiency increased in the following order: R-CTAB > GAC > R-H > PP > CF. The increase in operating time up to 300 min led to a decrease in oil removal efficiency for most materials. At the beginning, GAC had the best oil removal efficiency of 92.6%. However, with the increase of operating time, its efficiency decreased remarkably. Same tendency with PP and R-H was observed. On the contrary, the oil removal of R-CTAB and CF did not change too much with increasing time. Certainly, the oil removed by R-CTAB was more than 2 times that removed by CF with the oil removal efficiency of 81.2% and 36.0%, respectively. It can be concluded that the modified resin R-CTAB could purify oily wastewater much better than other investigated materials. Therefore, more attention was paid

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Figure 3. Effect of flow velocity on oil removal. Bed height ) 30 cm; pH ) 7.0; 30 °C.

Figure 4. Effect of the bed height on oil removal. Flow velocity ) 150 mL/h; pH ) 7.0; 25 °C.

to operating conditions for improving the treatment performance of R-CTAB in subsequent experiments. The effects of flow velocity, bed height, oil concentration (Ce), temperature, and pH value were studied. The feed oil concentration was 300 mg/L except the study of the effect of influent oil concentration. 3.2. Effect of the Flow Velocity. The influence of flow velocity on the oil removal is illustrated in Figure 3, presenting the dependence of the oil removal on all four flow velocities of 60, 120, 180, and 240 mL/h, respectively. It is obvious that with the increase of flow velocity, the oil removal decreased though that was not so remarkable below the flow velocity of 180 mL/h. Under the conditions mentioned above, the average removal rate of oil was 94.7, 88.1, 83.7, and 48.6%, respectively, and the filter medium performed better at low flow velocity. The negative charge of oil droplets was neutralized when contacted with the grafted quaternary ammonium cation and subsequent coalescence of the droplets took place due to interdroplet forces. A lower flow velocity, which means a longer retention time in the bed, is responsible for the coalescing process.22 It should be noted that even with the high flow velocity, for example, 240 mL/h, the oil removal was low but still stable (42-53%), not changing with the running time too much. That means no breakthrough happened even under high flow velocity. 3.3. Effect of the Bed Height. The effect of bed height on the effluent oil concentration and oil removal is presented in Figure 4. During the test, the flow velocity was kept at 150 mL/h. As can be seen, when the bed height was shorter than 20 cm, the operation was unstable and the outlet oil concentration increased. Short bed height resulted in short contact time that was insufficient for the drops coalescence. On the contrary, an increase in the bed height, which was directly related to valid column working volume, made a better

Figure 5. Effect of oil concentration on oil removal. Flow velocity ) 100 mL/h; bed height ) 30 cm; pH ) 7.0; 30 °C.

oil removal. As explained in section 3.2, longer bed height made a longer retention time, which was beneficial to O/W emulsion separation. However, as the bed height increased, higher inlet pressure was needed to maintain a certain flow velocity of the feed emulsion. In addition, when bed height was over a specific value (20 cm in this case), only slight changes in oil removal were found. Hence, in conclusion, the bed height has a significant influence on oil removal and a suitable bed height should be chosen for the optimum operation. 3.4. Effect of the Influent Oil Concentration. Emulsion influents with different oil concentrations of 50, 100, 300, and 500 mg/L were prepared to investigate the effect of influent oil concentration on column coalescer at 30 °C. Figure 5 shows the coalescer separation efficiency obtained with a 30 cm bed height and flow velocity of 100 mL/h. No noticeable decrease in oil removal occurred as expected within experimental data deviation, even though the oil concentration decuple increasing from 50 to 500 mg/L. In contrast, when emulsions of 500 mg/L oil concentration were treated, a clear improvement in separation efficiency was obtained compared to the more dilute emulsions during the operating period. There is an increased number of oil droplets present in the system at higher feed concentration and consequently more oil droplets present inside the thin flow channels of the column. A higher concentration of oil droplets inside the channels should lead to better coalescence. At higher concentrations, the probability of oil droplets colliding with each other or colliding with the pore surface increases, providing more chances for coalescence taking place. 3.5. Effect of Temperature. Bed height, flow velocity, and feed concentration were kept constant in a set of experiments for studying the effect of temperature on the performance. In order to observe the effect of temperature on oil removal, studies were carried out with modified resin columns at five different temperatures (20, 30, 40, 50, and 60 °C) and the results obtained are showed in Figure 6. An increase of temperature from 20 to 60 °C led to a modest improvement of oil removal efficiency up to about 85% for modified resin column. Weak changes in oil removal were found and an increase in oil removal was observed at high temperature, although the best performance seemed to be achieved at the intermediate temperature (50 °C), as shown in Figure 6. High temperature generally enhances coalescence, due to the lower fluid viscosity and the rise in the number of collisions between droplets, since their mobility is increased. Changes in density and viscosity of fuel oil as a function of temperature were investigated by Nordvik.23 Based on his data, we found that the density of both water and oil can be lowered with

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Figure 6. Effect of temperature on oil removal. Bed height ) 30 cm; flow velocity ) 120 mL/h; pH ) 7.0.

concentration decreased to the minimum of 30 mg/L, corresponding with a high oil removal of 90%. The results show that the acidic or alkali condition is more effective for oil removal than the neutral one. Acidification or acid breaking is a common process in breaking emulsion due to the electrical neutralization destabilization. As for the better oil removal efficiency in alkali condition, that is because of the impurities and additives present in the diesel oil such as organic nitrate acting as cetane number improver.24,25 These kinds of materials could react with sodium hydroxide and are saponified subsequently. During the alkali experiment, some buff floc formation was observed in the column bed, which indicated that the buff floc played an essential role in the emulsion breaking process. However, for safe discharge and avoiding excess consumption of chemicals, it is preferred to operate the unit in neutral condition. 4. Conclusions

Figure 7. Effect of pH on oil removal. Bed height ) 30 cm; flow velocity ) 100 mL/h; 25 °C.

increasing temperature, and the density difference of the two liquid is increasing with temperature increase. Thus, the net effect on the buoyancy force increases too, while the viscosity of fuel oil decreases. The changes of the two properties are both favorable for oil and water separation. So the oil droplets in the layer on the resin particle surface can well accommodate other droplets, and therefore the layer could move and desorb faster than that at lower temperature. However, the probability for a droplet to be entrapped is reduced at higher temperatures. An increase in temperature also leads to less favorable adsorption equilibrium. 3.6. Effect of the pH Value. Figure 7 shows the residual oil concentration and oil removal at the same conditions except pH. The pH was adjusted by hydrochloric acid or sodium hydroxide (1.0 mol/L) during experimental process. In aqueous media, the pH value is one of the most important factors that affects the emulsion’s zeta potential. Due to SDBS, an anionic surfactant, used as stabilizers in this study, a negatively charged surface of oil droplet was obtained. The zeta potentials of emulsions as a function of pH under same solution conditions were measured. The zeta potential of emulsion ranged from -15.8 mV (pH 2) to -78.1 mV (pH 10), being more negative with pH increase. It seems that the emulsions should be more stable and inefficient oil removal performance obtained in alkali condition due to the higher repulsive interaction. However, the best oil removal performances were obtained in alkali condition. As shown in Figure 7, oil removal efficiency differed evidently with the pH of feed wastewater. With pH value ranged at 5-8 the oil removal efficiency was about 81-84%, and better results were apparent beyond this range. At pH 2, the effluent oil concentration reduced to 40 mg/L with the oil removal of 86.6%; when more alkali added and pH further increased to 10, a lower effluent oil concentration was obtained. The residual oil

In this study, the application of modified resin as a filter medium treating the anionic surfactant stabilized oil-in-water emulsion was conducted. It was found that modified resin column coalescer achieved an effective oil separation comparing with GAC, PP, and CF. The results showed that the potential for using cationic surfactant modified resin to coalesce fine oil droplets in stable oily emulsions is significant. The need for expensive specific chemicals is eliminated and separation efficiency over 80% can be achieved under a wide range of operating conditions. The efficiency of the coalescence process applied was found to be a function of the flow velocity, bed height, temperature, influent oil concentration, and influent pH. Among these operating parameters, flow velocity and bed height that related to retention time play important roles in performance. Results indicated that there was an optimal flow velocity and bed height so that it could enhance droplet coalescence and avoid materials and energy squandering away. Controlling other parameters such as pH, oil concentration, and temperature that related to emulsion properties is also helpful for the coalescence process. An understanding of both chemical and physical separation mechanisms involved in the process is essential for proper filter medium selection and operational optimization in the future. Acknowledgment Financial support provided by the National Natural Science Foundation of China (No. 50578170) and by the Shanghai Leading Academic Discipline Project (No. B506) is gratefully acknowledged. Literature Cited (1) Guerin, T. F. Heavy Equipment Maintenance Wastes and Environmental Management in the Mining Industry. J. EnViron. Manage. 2002, 66, 185. (2) Morales Chabrand, R.; Kim, H. J.; Zhang, C.; Glatz, C. E.; Jung, S. Destabilization of the Emulsion Formed during Aqueous Extraction of Soybean Oil. J. Am. Oil Chem. Soc. 2008, 85, 383. (3) Cambiella, A.; Benito, J. M.; Pazos, C.; Coca, J. Interfacial Properties of Oil-in-water Emulsions Designed to be Used as Metalworking Fluids. Colloid Surf. A: Physicochem. Eng. Asp. 2007, 305, 112. (4) National Environmental Protection Agency. Integrated Wastewater Discharge Standard GB8978-1996, 1997, China. (5) Cheryan, M.; Rajagopalan, N. Membrane Processing of Oily Streams. Wastewater Treatment and Waste Reduction. J. Membr. Sci. 1998, 151, 13.

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ReceiVed for reView August 09, 2008 ReVised manuscript receiVed November 12, 2008 Accepted November 20, 2008 IE8012242