COALESCENCE AND SIZE DISTRIBUTION CHARACTERISTICS OF OIL DROPLETS ATTACHED ON FLOCS AFTER COAGULATION

COALESCENCE AND SIZE DISTRIBUTION CHARACTERISTICS OF OIL DROPLETS ATTACHED ON FLOCS AFTER COAGULATION BERRIN TANSEL and ORHAN SEVIMOGLU Civil and Environmental Engineering Department, Florida International University, Miami, FL 33174, U.S.A. ( author for correspondence, e-mail: Tanselb@fiu.edu) (Received 12 August 2003; accepted 1 September 2005) Abstract. The coalescence characteristics of oil droplets which are attached on flocs after coagulation is different from coalescence of droplets which are suspended in emulsions. The droplets attached on flocs are stationary and do not collide as those in emulsions. Objectives of this study were to investigate the change in size distribution of oil droplets that were attached on flocs after coagulation. The surface water oil emulsion was prepared by mixing water, clay and ethyl benzene. Flocculation/coagulation experiments were conducted using standard jar test procedure and a polyelectrolyte. Microscopic images of flocs were taken at different times after the flocculation process and analyzed to characterize the changes in droplet size distribution as a result of coalescence and detachment of droplets from the flocs. Median droplet size increased during the first 40 h after the flocculation process and decreased after 45 h due to detachment of droplets from flocs. The number of droplets that were larger than 90 µm decreased over time. After 46 h, the flocs had very few oil droplets remaining attached and a significant fraction of the flocs settled to the bottom. Although the coalescence rate of oil droplets on flocs was slow, for oil water separation applications, flocs should be removed from the solution as soon as possible to achieve higher separation efficiency of oil from the emulsion. Keywords: coagulation, coalescence, droplet distribution, emulsions, flocculation, image analysis 1. Introduction Emulsions are mixtures of two or more immiscible liquids where one liquid is present in the other in the form of droplets. Oil droplets in oil water emulsions can remain in suspension for a relatively long time depending on their surface charges, specific gravity, surface tension and solubility characteristics relative to water. Coagulation and flocculation processes have been used for separation of oil from oil water emulsions either as the main technology or as a pretreatment method for other water treatment technologies such as dissolved air flotation or membrane processes (Gray et al., 1997; Zunan et al., 1995; Plucinski and Reitmer, 1997; Tansel et al., 1995; Tansel and Eifert, 1999). Coagulants are typically solutions of salts with large ionic strength (i.e., aluminum and ferric) or polymers which attract colloidal particles or oil droplets causing them to agglomerate on flocs. Flocculation is often preceded by coagulation where a stable system of colloidal suspension is disturbed by the addition of a chemical (coagulant). Effectiveness of coagulant depends on the concentration of colloidal particles in the solution. If the Water, Air, and Soil Pollution (2006) 169: 293 302 C Springer 2006

294 B. TANSEL AND O. SEVIMOGLU Figure 1. Behavior of droplets attached on flocs after coagulation. concentration of the colloidal particles is too low, the aggregation process to form the flocs may not be effective. The role of coagulant in the coagulation process to disturb the stability of the suspended colloidal particles. When coagulants are added, the concentration of positively charged ions in the diffuse layer around the colloidal particles increases, leading to a compression of the diffuse double layer and smaller electrostatic repulsive forces between approaching particles. In addition, the colloidal particles can move towards each other due to dispersion or Van der Walls forces and coagulation ultimately occurs when the repulsion between the particles is eliminated and every collision results in particles contact (Basaran et al., 1998). When the turbidity of the surface water is due to the presence of clay, coagulants react with the clay particles and form clay polyelectrolyte complexes containing higher organic matter and, therefore, have greater affinity for hydrophobic organic compounds. The removal efficiencies of the hydrocarbons by coagulation have been observed to increase with increasing molecular size (Tansel and Eifert, 1999). Coalescence of oil droplets in emulsions occurs when droplets come into contact with each other. The interface between the droplets distorts and forms a flat lamella which ruptures leading to coalescence. After the coagulation, if the flocs are not removed from the solution immediately, the oil droplets which are concentrated on the flocs, coalesce and detach from the flocs as shown in Figure 1. A number of theories exist to describe the coalescence process. Based on the theory of droplet lifetime, oil-in-water droplets approaching from below a water oil interface show a minimum lifetime at droplet radius between 10 and 100 µm (Ivanov et al., 1997). The coalescence of the droplets which are already attached on the flocs is different from coalescence of droplets in emulsions. The droplets on flocs are stationary and do not collide as those in emulsions. The interaction energy of the droplets which are attached on the flocs are smaller and some droplets are separated by the floc matrix. On flocs, where droplets are aggregated, the gradient of interfacial tension on the interface of the droplets results in gradual drainage of lamella leading to coalescence. As a result, the size distribution of oil droplets which are attached on the flocs changes over time. Droplets grow in size until they become too large to

COALESCENCE AND SIZE DISTRIBUTION CHARACTERISTICS OF OIL DROPLETS 295 stay attached to the floc matrix. Detachment of the larger droplets from the flocs result in decrease in oil content and changes the floc density. If the oil has a lower density than water, typically the flocs would also have lower densities than water (due to aggregation of oil droplets on the floc). As a result, initially, flocs tend to concentrate on the surface forming a scum layer. As the coalescence progresses, droplets detach from the flocs, gradually increasing the floc density. Eventually, the flocs settle to the bottom as their densities exceed that of water. A number of droplet size distribution methods have been used to characterize coalescence of droplets in emulsions. Mishra et al. (1998) used a phase Doppler anemometer to measure the droplet size distribution as a function to time, shear rates, and sodium chloride concentration. Koh et al. (2000) used optical microscopy and photon correlation spectroscopy techniques to measure stability of oil-in-water emulsions in the presence of sodium chloride and surfactant. Rios et al. (1998) studied demulsification rates of oil water emulsions in a temperature range from 20 to 80 C using calcium chloride and aluminum chloride as electrolytes using photon correlation spectroscopy. Sæther et al. (1998) investigated the coupling of flocculation, coalescence and floc fragmentation in oil water emulsions. Basaran (1998) used an ultrasonic imaging technique to monitor gravitational separation in oil water emulsions. Microscopy techniques have been used in analysis of colloidal suspensions, particles, and aggregated sediments to examine the characteristics of particles (Hiemenz, 1986; Droppo et al., 1996, 1997). Application of image analysis techniques for analysis of microscopic images of flocs formed in surface water oil emulsions can reveal significant information about the characteristics of flocs, morphology and stability of the floc matrix, and interactions between droplets and flocs. Objectives of this study were to investigate the change in size distribution of oil droplets on flocs which were formed in surface water oil emulsions after the addition of a polyelectrolyte. Flocculation/coagulation experiments were conducted using standard jar test procedure. Microscopic images of flocs captured at different times after the flocculation were examined and statistically analyzed to determine the changes in the droplet size distribution as a result of coalescence and detachment of oil droplets from the flocs. 2. Materials and Methods Coagulation and flocculation experiments were conducted using a standard jar test unit Model PB-700 by Phipps and Bird Inc., Richmond, Virginia. Since coagulants are most effective when colloidal particles are present in water, clay was used to increase the concentration of colloidal particles. Four L of artificial surface water was prepared by adding clay (H 2 Al 2 Si 2 O 8 H 2 O) to tap water to have a final clay concentration of 16.7 mg/l, to provide colloidal characteristics similar to that of surface water. The solution was mixed for 30 min. The final turbidity of the clay water

296 B. TANSEL AND O. SEVIMOGLU mixture was 25 NTU, similar to that of surface water. Four ml of ethylbenzene (C 8 H 10 )was added to the artificial surface water to achieve a final oil concentration of 0.1% in the mixture. The solution was stirred for 40 min in a closed container to form the surface water oil emulsion. A series of coagulant screening tests were conducted using the standard jar test procedure to select an effective polyelectrolyte and to determine the optimum polyelectrolyte dosage. Screening process was conducted based on the formation of stable flocs and residual turbidity remaining in the solution at the completion of the jar test procedure. Each jar test was conducted with 1000 ml sample. The solution was stirred rapidly at 300 rpm for 2 min after coagulant addition, followed by slow mixing at 30 rpm for 15 min. The turbidity measurements and floc evaluations were conducted after 10 min with no mixing. After the screening experiments, Cat floc 2953 by Nalco Chemical Company, Naperville, Illinois was selected as the polyelectrolyte. Cat floc 2953 is an acidic coagulant with dimethyldiallyl ammonium as the active ingredient. The polyelectrolyte stock solution was prepared by mixing 1 ml of Cat floc 2953 in 100 ml deionized water for ease of handling, per manufacturer s suggestion. Based on the coagulant screening test results, the optimum dosage Cat floc 2953 was determined as 0.05 ml/l. Subsequent flocculation experiments were conducted by using the artificial surface water oil emulsion which was coagulated with the optimum polyelectrolyte dosage. Floc samples were taken for microscopic examination at times 0, 4, 9, 18, 22, 46 and 70 h after the coagulation flocculation process. For microscopic examination and image analysis, 1 2 drops of flocculated water samples were placed on the microscope slides using Pasteur pipettes with a large tip to minimize breakage of flocs during transfer. The flocs were examined by an Olympus System Microscope, model BX40, Olympus America Inc., Melville, NY. The microscope was equipped with a Sony Color video camera model DXC- 107A/107AP. The captured images were processed by Image-Pro Plus software by Media Cybernetics, Silver Spring, Maryland (Image-Pro Plus, 1997). The digital images were analyzed in the form of rectangular grids of 640 480 pixels corresponding to an image area of 414.26 µm 312.65 µm. Enhancement techniques ranging from simple operations such as brightness and contrast adjustment to complex spatial and morphological filtering operations were used as necessary to improve and refine visual information. The oil droplets were distinguished by color threshold. For each sample, between 50 and 60 flocs were examined to determine the typical floc characteristics. 3. Results and Discussion The flocs which were formed in the surface water ethylbenzene (oil) emulsion after the polyelectrolyte addition were microscopically examined. The number and size of oil droplets which were attached to the flocs were characterized statistically from

COALESCENCE AND SIZE DISTRIBUTION CHARACTERISTICS OF OIL DROPLETS 297 the microscopic images of flocs using Image Image-Pro Plus software. Changes in droplet size distribution and image characteristics were examined in relation to time. During the microscopic examinations, it was observed that the majority of the larger oil droplets aggregated on the floc surfaces and smaller droplets were attached within the floc matrix. Figure 2 shows the typical microscopic images of the flocs formed in the surface water oil emulsion immediately after the completion of the jar test (time = 0), after 22 h, and after 70 h. Immediately after the completion jar test, the flocs were observed to move to the water surface. The average number of droplets attached on flocs decreased by about 50% between 0 and 4 h. Since the gravitational force of the larger oil droplets is greater than the forces within the floc structure to keep larger droplets attached on the floc, the larger droplets were released from the flocs. After 46 h, as a result of coalescence and detachment, some flocs were observed to settle to the bottom of the solution. The examination of the flocs that settled to the bottom showed that these flocs did not have significant number of oil droplets but consisted of mainly clay particles. After 70 h, there were very few flocs remaining on the surface. These flocs had similar characteristics to the flocs which settled. Figure 3 presents the change in the average number of droplets of flocs over time in relation to droplet diameter. The flocs sampled at time = 0 had a large number of oil droplets which were less than 20 µm indiameter. Figure 4 presents the change in size distribution of droplets over time. After 4 h, the number of droplets with diameters between 50 and 90 µm decrease due to detachment and after 10 h increased due to coalescence of the remaining droplets. The number of droplets within the size ranges of 0 20 µm and 10 20 µm increased after 18 h. The number of droplets which were larger than 90 µm consistently decreased over time. This observation indicates that the droplets which were larger than 90 µm in diameter could not have a strong attachment to the floc matrix. An analysis of the droplet size distribution curves revealed that the histograms for the droplet size distribution at a specific time could be represented by the following exponential distribution function: f (x) = λe λx where λ:exponential function parameter; x: droplet size, microns; f(x): fraction of droplets with droplet size x. The exponential distribution function, from a statistical perspective, is used for situations where a system at state A changes to state B with constant probability per unit time λ. However, in this application λ was time dependent and decreased over time as shown in Figure 5. The time dependency of λ also showed an exponential correlation as follows: λ(t) = ae bt where a: initial value of λ at t = 0; b: rate constant (h 1 ).

298 B. TANSEL AND O. SEVIMOGLU Figure 2. Microscopic images of flocs taken at t = 0, t = 22, t = 70 h after the jar test.

COALESCENCE AND SIZE DISTRIBUTION CHARACTERISTICS OF OIL DROPLETS 299 Figure 3. Average number and size of oil droplets attached on flocs over time. For the ethyl benzene and water system studied, the values of a and b were determined as 0.0626 and 0.0213 per h as shown in Figure 5. Hence, the number of droplets at state A changed (i.e., decreased) to state B with constant probability over shorter and shorter periods of time λ which is given by the following time. λ(t) = 0.0626 e 0.0213t Figure 6 presents the change of median, 25 and 75 percentile droplet sizes over time. There were no significant changes in diameters of the smaller droplets which were below the 25 percentile. During microscopic viewing, the smaller droplets were observed to be entrapped within the floc matrix and could not easily come into contact with other droplets to coalesce. Median droplet size increased during

300 B. TANSEL AND O. SEVIMOGLU Figure 4. Droplet size distribution on flocs over time. Figure 5. Variation of statistical distribution parameter (λ) for droplet size over time. Figure 6. Size distribution characteristics of droplets attached on flocs.

COALESCENCE AND SIZE DISTRIBUTION CHARACTERISTICS OF OIL DROPLETS 301 the first 40 h after the flocculation process due to coalescence and decreased after 45 h due to detachment of the larger droplets as shown in Figure 6. 4. Conclusions Microscopic images of flocs were analyzed to understand the attachment and size distribution characteristics of oil droplets attached on flocs formed in oil water emulsions after coagulation. Microscopic examination of flocs revealed that oil droplets were removed from the emulsion by entrapment and adsorption within the floc matrix and on the floc surface. The droplet size distribution on flocs over time showed that droplets with diameters between 0 and 50 µm coalesce slowly to form larger droplets which eventually detach from the floc, increasing the floc density. The change in droplet size distribution on the flocs showed that the coalescence rate of the droplets was slow for the oil water emulsion prepared using ethylbenzene as the oil phase. After 46 h, some flocs settled due to the detachment of oil droplets from the floc surface. The larger oil droplets continuously detached from the floc surface after the coagulation. Although the coalescence rate of droplets on flocs was slow, for oil water separation applications, flocs should be removed from the solution as soon as possible to achieve higher separation efficiency of oil from the emulsion. References Basaran, T. K., Demetriades, K. and McClements, D. J.: 1998, Ultrasonic imaging of gravitational separation in emulsions, Colloids Surf. A: Physicochem. Eng. Aspects 136, 169 181. Droppo, I. G., Flanigan, D. T., Leppard, G. G., Jaskot, C. and Liss, S. N.: 1996, Floc stabilization for multiple microscopic techniques, Appl. Environ. Microbiol. 62, 3508 3515. Droppo, I. G., Leppard, G. G., Flannigan, D. T. and Liss, S. N.: 1997, The freshwater floc: A functional relationship of water and organic and inorganic floc constituents affecting suspended sediment properties, Water Air Soil Pollut. 99(1 4), 43 54. Gray, S. R., Harbour, P. J. and Dixon, D. R.: 1997, Effect of polyelectrolyte charge density and molecular weight on the flotation of oil in water emulsions, Colloids Surf. A: Physicochem. Eng. Aspects 126, 85 95. Hiemenz, P. C.: 1986, Principles of Colloid and Surface Chemistry, 2nd edn., Marcel Dekker, New York, 815 pp. Image-Pro Plus: 1997, Image-Pro Version 3 for Windows, Media Cybernetics, Silver Spring, MD. Ivanov, I. B. and Kralchevsky, P. A.: 1997, Stability of emulsions under equilibrium and dynamic conditions, Colloids Surf. A: Physicochem. Eng. Aspects 128, 155 175. Koh, A., Gillies, G., Gore, J. and Saunders, B. R.: 2000, Flocculation and coalescence of oil-in-water poly(dimethylsiloxane) emulsions, J. Colloid Interface Sci. 227, 390 397. Mishra, V., Kresta, S. M. and Masliyah, J. H.: 1998, Self-preservation of the drop size distribution function and variation in the stability ratio for rapid coalescence of a polydisperse emulsion in a simple shear field, J. Colloid Interface Sci. 197, 57 67.

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