Investigating Separation Efficiency of Oil-in-Water Emulsions Subjected to an Acoustic Field

AbstractThe demand for improved technologies that enhance the separation of low concentrations of oil from water and thus reduce the increasing retention times in surface separation facilities is increasing. Acoustophoresis is a promising technique where acoustic direct radiation forces can aid to demulsify O/W emulsions and enhance oil separation of the produced water stream. Herein, we explored the coalescence behavior of oil-in-water emulsion in a stationary acoustophoretic setup. To quantify the acoustic O/W separation efficiency and identify the proper acoustic parameters for various O/W emulsion compositions, a series of experiments were conducted in large-scale (4-in × 4in × 12-in) acoustic resonator using O/W emulsions of different compositions. The separation efficiency is assessed by comparing the oil layer thickness of an emulsion separated by gravity alone with that separated after being subjected to the standing wave field. The mixing time and speed was 15 minutes and 2000 rpm respectively which was optimized experimentally for stable emulsion. Rushton impeller was used for high turbulence mixing. Results demonstrated the importance of optimizing the acoustic parameters (frequency, power) with respect to the emulsion droplet size distribution for improving the separation efficiency. The formation of bands and the accelerated separation of oil droplets are facilitated by the application of sufficient energy to the proper standing wave. This study showed that when ultrasonic was applied to the emulsion under limited frequency and power, coalescence was shown on the surface, meaning in a standing wave field, oil droplets aggregate and collide in the anti-nodal planes, where their coalescence and buoyancy occur when direct radiation force (Aggregating oil droplet), and secondary acoustic force (when coalescence takes place due to the causing of attractive or repulsive forces), therefore separation will happen. Results showed that the oil layer thickness recovered from the O/W emulsion subjected to the acoustic field was ~70% higher than that of gravitational separation alone. Results also showed that deviating from the standing wave frequency or delivering excessive acoustic power can result in random droplet motion, secondary emulsification, and a decrease in separation efficiency. An observation experiment was done using stable emulsions with droplet sizes ranging from 50 to 100 micron subjected at ultrasonic radiation with varying amount of acoustic power starting with small amount of powerand increased gradually until reach the maximum power; 111 to 280 watts, showedemulsion separation is progressing with maximum efficiency or standing wave is creating the maximum capacity for an emulsion dispersed content. This regime of system depends critically on the oil content of an emulsion. Also, computational work has been conducted using COMSOL Multiphysics for illustration of frequency influence on oil separation. This work provides novel information to direct the field implementation of in-line acoustic oil-water separation tool by identifying the key parameters that influence oil coalescence hence the separation efficiency of the tool.