**3. Crude oil emulsions**

In order to promote the effective mixing between the organic and aqueous phases, and ensure the proper dilution of the salts and minerals in the aqueous phase, a mixing valve is used. This is a common globe valvewhich causes a pressure drop and, as a result, a shear stress over the droplets that promotes an intimate water and oil mixture. The main aspect that needs to be considered is the pressure drop, whose value it is about 10-50 psi and varies according to the flux through the valve (automatic differential pressure controllers could be used). In addition to the mixing valve, upstream premixing device could be used, such as spray nozzles at the point of water injection or static mixers, between the water injection point and the mixing valve [4]. High delta pressure in the mixing valve promotes smaller droplets, which is positive because it improves the contact among the phases; however, very small droplets could yield a more stable emulsion, which could cause problems in the separating vessel. Therefore, it is

very important to balance both effects in the selection of the operation pressure drop.

force (*F*) between the water droplets is given by:

Ks is a constant for the system [4].

70 Advances in Petrochemicals

by:

Then this mixture goes to the desalter, a horizontal cylindrical tank that provides long enough residence time to separate the water and oil mixture in two phases. Some water droplets diameters are so small that they could not be separated by gravity; so, an electrostatic field between two electrodes installed into the desalter is used to promote coalescence. Due to the dipolar nature of the water molecule, this electric field promotes an attraction with the other water molecules in the neighbor droplets promoting coalescence. According to Gary et Al. [2], either AC or DC fields may be used and potentials from 12, 000 to 35, 000 volts. The attraction

> 2 6 4 *K d <sup>s</sup> <sup>F</sup> s* × × e

where ε is voltage gradient, *d* is droplet diameter, *s* is the distance between drops centers and

Finally, after coalescence, water droplets settle according to the well-known Stock's law given

r

In a one-step configuration, shown in Figure 1(a), 90% of salt removal can be achieved. For higher salt removal percentages, a two-step configuration, shown in Figure 1(b), should be required. As far as the last process configuration is concerned two demulsifier injection points are used, both before the mixing valve in the first and second step. In addition, freshwater is fed to the second stage, and effluent water from this stage is recycled to the first one. With this

where *k* is a constant, d is the droplet diameter, *ρ* is density, and *μ* is viscosity.

*Settling rate*

configuration, a 99% salt removal could be achieved.

( ) <sup>2</sup> *water oil oil k d*

m

 r

= (1)

× -× <sup>=</sup> (2)

Emulsions are present in the main operations of the petroleum industry. The more wellknown emulsions in the petroleum industry are those when the crude oil is the external phase (W/O). Water-in-oil emulsions are robustly stabilized by natural surfactants from crude oil, where a "special film" takes place and it is the responsible for emulsion stability. Film at the oil–water interface is highly viscous and is formed by asphaltenes, resins, waxes, and small solid particles [5, 6].

A main feature for interfacial film is the irreversible adsorption of asphaltenes at the oil−water interface in combination with other natural surfactants [7-10].

In the study of water-in-oil emulsions, it is important to know the structure and properties of the crude oil components, their tendency to associate and accumulate at the interface, as well as solubility and sensitivity to pressure and temperature changes.
