Abstract
Dispersed water drops in oils can cause detrimental effects. For example in refining and cracking of crude oil, poisoning of catalysts and equipment corrosion can be major problems due to the presence of water. Therefore there is a need to develop more efficient and cost-effective separators. The work in this thesis covers an investigation of the underlying electrostatic phenomena (i.e. drop-drop and drop-interface coalescence and drop deformation and break-up) leading to the development of compact electrocoalescer-separators. Using novel two-dimensional electrode systems, it is shown here that coalescence readily occurs when the electric field is applied in the same direction as the line joining the centres of the two drops, in line with the previous theoretical prediction of the maximum attractive force induced in this way. An adaptive Finite Element Method, incorporating automatic error and mesh generator programmes, has been used to quantify the electric-induced forces of charged spheres in an insulating medium, for short distances that are much smaller than the drop radius, where analytical solutions are no longer reliable. Moreover, drop-drop attraction can also occur when the angle is 125.3° from the electric field direction. Previous work suggests that pulsed d.c. fields are effective for low-aqueous-content systems, and this has been further investigated here. The applied electric field and pulse frequency can be optimised to achieve the highest coalescence rate. High electric fields (> 3.5 kV/cm) are shown to deform and break up aqueous drops when the electric field-induced stresses overcome the interfacial tension. A critical electrostatic Weber number, corresponding to the onset of drop break-up, has been evaluated to be about 0.49 for several aqueous-oil systems. For the separation of dispersed aqueous drops from an oil phase, it is advantageous to incorporate an aqueous layer to induce drop-interface coalescence. A suitably applied electric field is shown to give efficient drop-interface coalescence. The above investigations have led to the design and development of two compact electrocoalescer-separators. The first separator combines the effects of electrocoalescence and gravitational settling. The second separator, which utilises the electrocoalescence and centrifugal effects, can handle larger throughputs with reasonable separation efficiency. Both devices significantly enhance the separation and are good examples of how the fundamental understanding gained in this studies can be applied to realistic practical situations.