Coalescing of the small water drops dispersed in the crude can be accomplished by subjecting the water-in-oil emulsion to a high-voltage electrical field. When a non-conductive liquid (oil) containing a dispersed conductive liquid (water) is subjected to an electrostatic field, the conductive particles or droplets are caused to combine by one of three physical phenomena:
1. The droplets become polarized and tend to align themselves with the lines of electric force. In so doing, the positive and negative poles of the droplets are brought adjacent to each other. Electrical attraction brings the droplets together and causes them to coalesce.
2. Droplets are attracted to an electrode due to an induced charge. In an A-C field, due to inertia, small droplets vibrate over a larger distance than larger droplets promoting coalescence. In a D-C field the
droplets tend to collect on the electrodes forming larger and larger drops until eventually they fall by gravity.
3. The electric field tends to distort and thus weaken the film of emulsifier surrounding the water droplets. Water droplets dispersed in oil and subjected to a sinusoidal alternating-current field will be elongated along the lines of force during the first half cycle. As they are relaxed during the low-voltage portion, the surface tension will pull the droplets back toward the spherical shape. The same effect is obtained in the next half of the alternating cycle. The weakened film is thus more easily broken when droplets collide, making coalescence more likely.
Whatever the actual mechanism, the electric field causes the droplets to move about rapidly in random directions, which greatly increases the chances of collision with another droplet. When droplets collide with the proper velocity, coalescence occurs.
The attraction between water droplets in an electric field is given by:
This equation indicates that the greater the voltage gradient the greater for the forces causing coalescence. However, experimental data show that at some gradient the water droplet can be pulled apart and a strong emulsion can be developed. For this reason electrostatic treaters are normally equipped with a mechanism for adjusting the gradient in the field.
