Forecasting the Problem of Excessive Oil Entrainment in a Desalter Using Spinning Drop Method.

Frequent desalter upsets in the refineries processing opportunity crude oils are often triggered by a rapid and uncontrollable buildup of the rag layer, a thick water-in-oil emulsion, at the oil-brine interface. This is caused by spontaneous emulsification of brine in oil. This study investigates a unique observation from a spinning drop (SD) tensiometer, revealing the low apparent interfacial tension and rigidity of SD caused by spontaneous emulsification. Fine droplets of brine generated through spontaneous emulsification decorate the SD surface and form a stable, low-energy bilayer. Simulated rag layers using the brines from upset incidences exhibit similar behavior, indicating that spontaneous emulsification is driven by chemical species in brine, which promote osmotic water transport. The rate of rag layer buildup correlates with the rate of spontaneous emulsification, with the temperature coefficient of interfacial tension reduction serving as a sensitive indicator. An imminent upset in the operation can be forecasted by measuring this temperature coefficient, enabling preventive measures.

Catastrophic drop breakup in electric field.

We report novel observations revealing the catastrophic breakup of water drops containing surfactant molecules, which are suspended in oil and subjected to an electric field of strength approximately 10(5) V/m. The observed breakup was distinctly different from the gradual end pinch-off or tip-streaming modes reported earlier in the literature. There was no observable characteristic deformation of the drop prior to breakup. The time scales involved in the breakup and the resultant droplet sizes were much smaller in the phenomenon observed by us. We hypothesize that this mode of drop breakup is obtained by the combined effect of an external electric field that imposes tensile stresses on the surface of the drop, and characteristic stress-strain behavior for tensile deformation exhibited by the liquid drop in the presence of a suitable surfactant, which not only lowers the interfacial tension (and hence the cohesive strength) of the drop but also simultaneously renders the interface nonductile or brittle at high enough concentration. We have identified the relevant thermodynamic parameter, viz., the sum of interfacial tension, sigma, and the Gibbs elasticity, epsilon, which plays a decisive role in determining the mode of drop breakup. The parameter (epsilon + sigma) represents the internal restoration stress of a liquid drop opposing rapid, short-time-scale perturbations or local deformations in the drop shape under the influence of external impulses or stresses. A thermodynamic "state" diagram of (epsilon + sigma) versus interfacial area per surfactant molecule adsorbed at the drop interface shows a "maximum" at a critical transition concentration (ctc). Below this concentration of the surfactant, the drop undergoes tip streaming or pinch off. Above this concentration, the drop may undergo catastrophic disintegration if the external stress is high enough to overcome the ultimate cohesive strength of the drop's interface.

"Micro-pottery"--marangoni effect driven assembly of amphiphilic fibers.

We report spontaneous supra-assembly of fibrous surfactant crystallites at the air-solution interface resulting in spectacular arrays of two-dimensional spiral and three-dimensional "micro-pottery"-like superstructures. Surface pressure differential driven bending of the embryonic fiber nuclei and Marangoni convection driven fiber migration/alignment appear to be the causal factors behind this phenomenon. The assemblies form at specific crystal-growth velocities dictated by the relative time scales for fiber bending/alignment and their rigidification/immobilization as they grow. Although our studies are restricted to a specific class of amphiphiles, namely, alkaline metal salts of linear fatty acids, the phenomenon should be generic to amphiphilic molecules that crystallize into flexible fibers.