Study On Freeze/Thaw Induced Demulsification Process Of W/O Pickering Emulsion

Emulsions stabilized by solid particles are called Pickering emulsions, widely existing in the field of oil field, food and pharmacy. Crude oil containing water is harmful to the transportation, equipment and refinery, so demulsification is one of the key steps in various sectors of the oil industry. Current investigations indicate that particles contribute to the emulsion stability, arising the difficult in demulsification processes of some emulsion systems. Freeze/thaw has achieved great results in treating some emulsion systems rich in particles as a physical demulsifying technology, attaining higher dehydration ratio compared to some traditional demulsifying methods. However, up to now, there is hardly any theoretical study on the demulsifying processes of Pickering emulsion through freeze/thaw method. Moreover, the understanding of the role behavior of particles play in the process is also necessary.In this work, montmorillonite (MMT)or silica (SiO2)were chosen as the solid particles to stabilize emulsions associating with surfactant alkenysuccinimide(T151) and series of emulsions of different water content, different MMT concentrations as well as different type of particles were prepared for investigation. The influence of particles on the emulsion stability were investigated via the characterization of emulsion properties, including the viscosity, dispersed phase freezing point(DPFP), interfacial tension and droplet size.Operating conditions consist of freezing conditions, freezing time and thawing conditions were varied to estimate the dehydration performance of freeze/thaw on treating different emulsion. In addition, the dehydration ratio of Pickering emulsions and normal emulsion were compared and particle-assist-demulsification mechanism in freeze/thaw process is put forward.The effect of the water content was investigated. The results show that viscosity of Pickering emulsion increases with the water content, and the DPFP moves to-28℃from-36℃influenced by droplet size and particles, with a sufficient freezing time (SFT) as short as 25h. Moreover, because of the highly pack density of droplet, the dehydration ratio of high water contentemulsion is larger than that of low water content emulsion. The combination of freeze/thaw and centrifugation can improve the dehydration ratio of low water content emulsion to 50%,65%,77%and 88%, respectively.The emulsions with different MMT concentrations were studied. It is shown that emulsion viscosity increase with particles concentration and the MMT particles adsorbed on the interface can speed the crystallization as nucleus, resulting in promoting the DPFP to-28℃--25℃and shorting the SFT to 25h, whereas the KCl in the dispersed phase will cause the DPFP down to as low as -42.5℃and the SPF prolonged to 55h. Emulsions stabilized solely by MMT particles can enhance the interfacial membrane viscoelasticity so much that the freeze/thaw stability of emulsion is high, with the dehydration ratio as low as 0.5%.Analysis based on the effects of water content and particle concentrations indicates that the particles can enhance the demulsification ratiof emulsion with water content less than 70%. Nevertheless, there is a critical particle concentration(CPC). When the particle concentration exceeds the CPC, the dehydration ratio will decline instead. And the lower the freezing temperature is, the lower the CPC is. Additionally, high freezing temperature can attain satisfactory dehydration ratio and the thawing temperature exerts no effect on the separation performance. Finally, the effect of particle properties on the demulsification process was further explored by comparing interfacial tension, DPFP and SPF. The results show that hydrophilic particles have stronger interaction with the dispersed phase than the hydrophobic particles do and will raise the emulsion stabilization. And there is no necessary connection between DPFP and the dehydration ratio.Particle-assist -demulsification mechanism is stated as follows:when the particle mainly adsorbed on the interface, the uneven particle distribution is responsible for the membrane defect and will modify the shape of the ice crystals, enhancing the partial coalescence of adjacent droplets; when most of the particles are in the continuous phase, the volume expansion of ice crystals will squeeze small water droplet trapped in cavity of the three dimensional particle network aided by solid particles.