OilInWater Emulsions

Part of the oil added to water is suspended as large droplets, which rise to the water surface and form an oil film. Another part forms very small droplets as a result of energy input, for example from a rotating stirrer, and is not able to rise to the surface because of the equilibrium between very low buoyancy forces and downward diffusion. They behave as large molecules and move very slowly. If they collide with other small droplets, they coalesce; and then the diameter of the droplets increases and the emulsion is destabilized.

A stabilized emulsion arises by adsorption of anions at the surface. These anions may already be dissolved as SOJ-, HCO-, Cl-, etc., or they may be added as special chemical compounds (emulsifiers). The following results are obtained by experiments with dodecane (not soluble in water) and the emulsifier Eumulgin ET 5 (Henkel AG). In a rotor-stator mixer at a speed of 10000 min-1 and a stirring time of 60 s, a stable emulsion was produced with the droplet size distribution presented in Fig. 2.4.

0.55 0.66 0.78 0.96 1.2 1.4 1.7 2 2.4 2.9 3.5 4.2 5

Fig. 2.4 Size distribution of dodecane droplets in an emulsion stabilized by an emulsifier (Eumulgin ET 5), SD0 = 750 mg L-1, stirrer speed n = 10000 min-1, cET5 = 150 mg L-1, mixing time = 60 s (Cuno 1996).

0.55 0.66 0.78 0.96 1.2 1.4 1.7 2 2.4 2.9 3.5 4.2 5

Diameter d

Fig. 2.4 Size distribution of dodecane droplets in an emulsion stabilized by an emulsifier (Eumulgin ET 5), SD0 = 750 mg L-1, stirrer speed n = 10000 min-1, cET5 = 150 mg L-1, mixing time = 60 s (Cuno 1996).

The diameters of the droplets were measured and the droplets were counted using a microscope (Cuno 1996). In this emulsion, nearly all droplets had diameters between 0.5 pm and 3.5 pm, with a maximum fraction of 25% at 1.5 pm. The fraction describes the number of droplets of this size related to the total number.

Table 2.5 presents some industrial effluents with different oil concentrations. Only a part of the oil concentrations mentioned is really emulsified, while a part is suspended as larger droplets and yet another part may exist as an oil film at the surface.

Table 2.5 Industrial oil/water suspensions and emulsions (Patterson 1985).

Wastewater production

Oil concentration (mg L-1 COD)

Steel rolling mill 7200

Aluminium rolling mill 5000-50 000

Ferrous casting 20-716

Food industry 3820

Fish processing 520-13 700

Production of vegetable oil 4000-6000

Washing water of airplanes 500-12000

Textile industry (cleaning of wool and threads) 1605-12 260

It is difficult to measure the concentration of emulsified oil. After separation of nearly all the suspended oil particles by sedimentation (ascending), the oil in the stabilized emulsion can be oxidized and measured as total oxygen demand (TOD), or COD for longer oxidation times. But if there are other dissolved organics, the emulsion must be separated in advance using nano- or ultrafiltration membrane or an extraction method (direct extraction with petrol ether; DIN 38409; Walter 1993).

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