COALESCENCE AND SIZE DISTRIBUTION CHARACTERISTICS OFOIL DROPLETS ATTACHED ON FLOCS AFTER COAGULATION

BERRIN TANSEL∗ and ORHAN SEVIMOGLUCivil 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 coagulationis different from coalescence of droplets which are suspended in emulsions. The droplets attached onflocs are stationary and do not collide as those in emulsions. Objectives of this study were to investigatethe change in size distribution of oil droplets that were attached on flocs after coagulation. The surfacewater–oil emulsion was prepared by mixing water, clay and ethyl benzene. Flocculation/coagulationexperiments were conducted using standard jar test procedure and a polyelectrolyte. Microscopicimages of flocs were taken at different times after the flocculation process and analyzed to characterizethe changes in droplet size distribution as a result of coalescence and detachment of droplets from theflocs. Median droplet size increased during the first 40 h after the flocculation process and decreasedafter 45 h due to detachment of droplets from flocs. The number of droplets that were larger than90 µm decreased over time. After 46 h, the flocs had very few oil droplets remaining attached and asignificant fraction of the flocs settled to the bottom. Although the coalescence rate of oil droplets onflocs was slow, for oil–water separation applications, flocs should be removed from the solution assoon 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 ispresent in the other in the form of droplets. Oil droplets in oil–water emulsionscan remain in suspension for a relatively long time depending on their surfacecharges, specific gravity, surface tension and solubility characteristics relative towater. Coagulation and flocculation processes have been used for separation of oilfrom oil–water emulsions either as the main technology or as a pretreatment methodfor other water treatment technologies such as dissolved air flotation or membraneprocesses (Gray et al., 1997; Zunan et al., 1995; Plucinski and Reitmer, 1997;Tansel et al., 1995; Tansel and Eifert, 1999). Coagulants are typically solutionsof salts with large ionic strength (i.e., aluminum and ferric) or polymers whichattract colloidal particles or oil droplets causing them to agglomerate on flocs.Flocculation is often preceded by coagulation where a stable system of colloidalsuspension is disturbed by the addition of a chemical (coagulant). Effectiveness ofcoagulant depends on the concentration of colloidal particles in the solution. If the

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Figure 1. Behavior of droplets attached on flocs after coagulation.

concentration of the colloidal particles is too low, the aggregation process to formthe flocs may not be effective. The role of coagulant in the coagulation processto disturb the stability of the suspended colloidal particles. When coagulants areadded, the concentration of positively charged ions in the diffuse layer around thecolloidal particles increases, leading to a compression of the diffuse double layer andsmaller electrostatic repulsive forces between approaching particles. In addition, thecolloidal particles can move towards each other due to dispersion or Van der Wallsforces and coagulation ultimately occurs when the repulsion between the particlesis 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, coagulantsreact with the clay particles and form clay–polyelectrolyte complexes containinghigher organic matter and, therefore, have greater affinity for hydrophobic organiccompounds. The removal efficiencies of the hydrocarbons by coagulation have beenobserved to increase with increasing molecular size (Tansel and Eifert, 1999).

Coalescence of oil droplets in emulsions occurs when droplets come into contactwith each other. The interface between the droplets distorts and forms a flat lamellawhich ruptures leading to coalescence. After the coagulation, if the flocs are notremoved from the solution immediately, the oil droplets which are concentrated onthe flocs, coalesce and detach from the flocs as shown in Figure 1. A number oftheories exist to describe the coalescence process. Based on the theory of dropletlifetime, oil-in-water droplets approaching from below a water–oil interface showa 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 differentfrom coalescence of droplets in emulsions. The droplets on flocs are stationary anddo not collide as those in emulsions. The interaction energy of the droplets whichare attached on the flocs are smaller and some droplets are separated by the flocmatrix. On flocs, where droplets are aggregated, the gradient of interfacial tensionon the interface of the droplets results in gradual drainage of lamella leading tocoalescence. As a result, the size distribution of oil droplets which are attached onthe flocs changes over time. Droplets grow in size until they become too large to

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stay attached to the floc matrix. Detachment of the larger droplets from the flocsresult in decrease in oil content and changes the floc density. If the oil has a lowerdensity 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 toconcentrate on the surface forming a scum layer. As the coalescence progresses,droplets detach from the flocs, gradually increasing the floc density. Eventually, theflocs settle to the bottom as their densities exceed that of water.

A number of droplet size distribution methods have been used to characterizecoalescence of droplets in emulsions. Mishra et al. (1998) used a phase Doppleranemometer to measure the droplet size distribution as a function to time, shearrates, and sodium chloride concentration. Koh et al. (2000) used optical microscopyand photon correlation spectroscopy techniques to measure stability of oil-in-wateremulsions in the presence of sodium chloride and surfactant. Rios et al. (1998)studied demulsification rates of oil–water emulsions in a temperature range from20 to 80 ◦C using calcium chloride and aluminum chloride as electrolytes usingphoton correlation spectroscopy. Sæther et al. (1998) investigated the coupling offlocculation, coalescence and floc fragmentation in oil–water emulsions. Basaran(1998) used an ultrasonic imaging technique to monitor gravitational separation inoil–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 tech-niques for analysis of microscopic images of flocs formed in surface water–oilemulsions can reveal significant information about the characteristics of flocs, mor-phology 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 oildroplets on flocs which were formed in surface water–oil emulsions after the ad-dition of a polyelectrolyte. Flocculation/coagulation experiments were conductedusing standard jar test procedure. Microscopic images of flocs captured at differenttimes after the flocculation were examined and statistically analyzed to determinethe changes in the droplet size distribution as a result of coalescence and detachmentof oil droplets from the flocs.

2. Materials and Methods

Coagulation and flocculation experiments were conducted using a standard jar testunit Model PB-700 by Phipps and Bird Inc., Richmond, Virginia. Since coagulantsare most effective when colloidal particles are present in water, clay was used to in-crease the concentration of colloidal particles. Four L of artificial surface water wasprepared by adding clay (H2Al2Si2O8·H2O) to tap water to have a final clay concen-tration of 16.7 mg/l, to provide colloidal characteristics similar to that of surfacewater. The solution was mixed for 30 min. The final turbidity of the clay–water

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mixture was 25 NTU, similar to that of surface water. Four ml of ethylbenzene(C8H10) was added to the artificial surface water to achieve a final oil concentrationof 0.1% in the mixture. The solution was stirred for 40 min in a closed container toform the surface water–oil emulsion.

A series of coagulant screening tests were conducted using the standard jartest procedure to select an effective polyelectrolyte and to determine the optimumpolyelectrolyte dosage. Screening process was conducted based on the formationof stable flocs and residual turbidity remaining in the solution at the completionof the jar test procedure. Each jar test was conducted with 1000 ml sample. Thesolution was stirred rapidly at 300 rpm for 2 min after coagulant addition, followedby slow mixing at 30 rpm for 15 min. The turbidity measurements and floc evalua-tions were conducted after 10 min with no mixing. After the screening experiments,Cat floc 2953 by Nalco Chemical Company, Naperville, Illinois was selected asthe polyelectrolyte. Cat floc 2953 is an acidic coagulant with dimethyldiallyl am-monium as the active ingredient. The polyelectrolyte stock solution was preparedby 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, theoptimum dosage Cat floc 2953 was determined as 0.05 ml/l. Subsequent floccula-tion experiments were conducted by using the artificial surface–water–oil emulsionwhich was coagulated with the optimum polyelectrolyte dosage. Floc samples weretaken for microscopic examination at times 0, 4, 9, 18, 22, 46 and 70 h after thecoagulation–flocculation process.

For microscopic examination and image analysis, 1–2 drops of flocculated wa-ter samples were placed on the microscope slides using Pasteur pipettes with alarge tip to minimize breakage of flocs during transfer. The flocs were examined byan 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 softwareby Media Cybernetics, Silver Spring, Maryland (Image-Pro Plus, 1997). The dig-ital images were analyzed in the form of rectangular grids of 640 × 480 pixelscorresponding to an image area of 414.26 µm × 312.65 µm. Enhancement tech-niques ranging from simple operations such as brightness and contrast adjustmentto complex spatial and morphological filtering operations were used as necessary toimprove and refine visual information. The oil droplets were distinguished by colorthreshold. For each sample, between 50 and 60 flocs were examined to determinethe typical floc characteristics.

3. Results and Discussion

The flocs which were formed in the surface water–ethylbenzene (oil) emulsion afterthe polyelectrolyte addition were microscopically examined. The number and sizeof oil droplets which were attached to the flocs were characterized statistically from

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the microscopic images of flocs using Image Image-Pro Plus software. Changesin droplet size distribution and image characteristics were examined in relation totime. During the microscopic examinations, it was observed that the majority ofthe larger oil droplets aggregated on the floc surfaces and smaller droplets wereattached within the floc matrix.

Figure 2 shows the typical microscopic images of the flocs formed in the surfacewater–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 wereobserved to move to the water surface. The average number of droplets attachedon flocs decreased by about 50% between 0 and 4 h. Since the gravitational forceof the larger oil droplets is greater than the forces within the floc structure to keeplarger 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 tosettle to the bottom of the solution. The examination of the flocs that settled to thebottom showed that these flocs did not have significant number of oil droplets butconsisted of mainly clay particles. After 70 h, there were very few flocs remainingon 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 timein relation to droplet diameter. The flocs sampled at time = 0 had a large number ofoil droplets which were less than 20 µm in diameter. Figure 4 presents the changein size distribution of droplets over time. After 4 h, the number of droplets with di-ameters between 50 and 90 µm decrease due to detachment and after 10 h increaseddue to coalescence of the remaining droplets. The number of droplets within thesize ranges of 0–20 µm and 10–20 µm increased after 18 h. The number of dropletswhich were larger than 90 µm consistently decreased over time. This observationindicates that the droplets which were larger than 90 µm in diameter could not havea strong attachment to the floc matrix. An analysis of the droplet size distributioncurves revealed that the histograms for the droplet size distribution at a specifictime 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 ofdroplets with droplet size x.

The exponential distribution function, from a statistical perspective, is used forsituations where a system at state A changes to state B with constant probability perunit time λ. However, in this application λ was time dependent and decreased overtime as shown in Figure 5. The time dependency of λ also showed an exponentialcorrelation as follows:

λ(t) = ae−bt

where a: initial value of λ at t = 0; b: rate constant (h−1).

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Figure 2. Microscopic images of flocs taken at t = 0, t = 22, t = 70 h after the jar test.

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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 weredetermined as 0.0626 and 0.0213 per h as shown in Figure 5. Hence, the numberof droplets at state A changed (i.e., decreased) to state B with constant probabilityover 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 overtime. There were no significant changes in diameters of the smaller droplets whichwere below the 25 percentile. During microscopic viewing, the smaller dropletswere observed to be entrapped within the floc matrix and could not easily comeinto contact with other droplets to coalesce. Median droplet size increased during

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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.

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the first 40 h after the flocculation process due to coalescence and decreased after45 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 sizedistribution characteristics of oil droplets attached on flocs formed in oil–wateremulsions after coagulation. Microscopic examination of flocs revealed that oildroplets were removed from the emulsion by entrapment and adsorption within thefloc matrix and on the floc surface. The droplet size distribution on flocs over timeshowed that droplets with diameters between 0 and 50 µm coalesce slowly to formlarger droplets which eventually detach from the floc, increasing the floc density.The change in droplet size distribution on the flocs showed that the coalescence rateof the droplets was slow for the oil–water emulsion prepared using ethylbenzeneas the oil phase. After 46 h, some flocs settled due to the detachment of oil dropletsfrom the floc surface. The larger oil droplets continuously detached from the flocsurface after the coagulation. Although the coalescence rate of droplets on flocswas slow, for oil–water separation applications, flocs should be removed from thesolution as soon as possible to achieve higher separation efficiency of oil from theemulsion.

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