Study of Electrostatic Oil/Water Separation by Ultrasound Monitoring: Comparison With Gravity Separation Process

Abstract

The increasing number of off-shore projects has resulted in an important R&D effort to optimize the vessel sizes used for electrostatic dehydrators and desalters, otherwise known as electrocoalescers (ECS). Despite the maturity of this technology, the separator design remains mostly empirical. A very important part of the ECS has an inlet distribution device in the bottom of the vessel: similar to the gravity vertical separators or "Wash Tanks" separators configuration. In this last process, it has been found that the water drop accumulation at the oil water interface in a compact layer, so-called Dense Packed Zone (DPZ), plays a crucial role in the separation efficiency because it acts as a coalescence promoter. The objective of this work is to investigate the oil/water separation under an electrostatic or gravity field, in order to establish the presence and role of the DPZ in the separation mechanisms. An open continuous loop pilot (DUSS), equipped with an ultrasound scanner and an electric tension generator, was used in this study. DUSS mimics the wash tank or ECS processes. Two crude oils with contrasted separation behaviors were studied. Results show that in order to achieve an optimal separation, it is important to control the inlet flow rates to form a stable DPZ with an optimal thickness, either by gravity or electrostatic separation. In the electrostatic case, the electric field placed into the oil zone improves the coalescence of smaller drops that are not able to settle under the gravity field. Then, some of these bigger droplets feed the DPZ, increasing its thickness and the overall separation efficiency. Nevertheless, the DPZ should not reach the electric field in order to avoid shortcuts. Over this configuration, it is possible to successfully apply a modeling approach used for vertical gravity separation, known as the Dispersion Band Model. The robust experimental data obtained provides an interesting base for the understanding of one of the most classical electrostatic separation processes, useful information for the process optimization.