Removing Water from Diesel Fuel: Understanding the Impact of Droplet Size on Dynamic Interfacial Tension of Water-in-Fuel Emulsions

Abstract

Small amounts of water can enter diesel fuel during usage, causing major damage and failure of engine parts. Water is dispersed in fuel as droplets stabilized by the presence of surface-active compounds in the original fuel mixture as well as in fuel additives, including lubricity improvers and deposit control agents. Additives partition to the fuel–water interface and lower the interfacial tension (IFT), decreasing the ability to coalesce and separate water from fuel. The ability of standard coalescing filters to capture and coalesce emulsion droplets depends on dynamic IFT, conventionally measured for large millimeter-sized drops or planar interfaces. In this work, a microfluidic platform is employed to generate a monodisperse stream of small micrometer-sized water droplets in model fuel and ultralow sulfur diesel, mimicking the size of droplets in actual fuel–water emulsions. The deformation of hundreds of droplets is tracked at high speed through twenty-six geometric contractions to find time-dependent apparent IFT. It is found that the time scale associated with the decrease of IFT is orders of magnitude smaller in micrometer-sized droplets compared to millimeter-sized drops from pendant drop experiments. This finding suggests that, in real emulsion processing conditions such as fuel filtration, the residence time of droplets from the point of formation to filtration is such that IFT has already decreased to the equilibrium value. This work results in clear implications that standardized tests used by industry for qualifying diesel fuels must be reconsidered to account for droplet size, to enable design of efficient fuel filtration systems.

Insights into the Microscale Coalescence Behavior of Surfactant-Stabilized Droplets Using a Microfluidic Hydrodynamic Trap

“…Moreover, several studies have shown that surfactant transport to curved interfaces is governed not only by the surfactant concentration in the bulk phase but also by the curvature or radius of the interface . With water-in-diesel fuel emulsions, we previously showed that diffusion of surfactant to a highly curved, micrometer-sized droplet in flow is faster than that to a planar interface, resulting in the transport shifting toward being limited by kinetics rather than diffusion . These observations lead to the conclusion that coalescence behavior in emulsions, which contain micrometer-scale droplets, could be influenced not only by the surfactant concentration but also by the surfactant transport to and at the interface.…”

“…29 With water-in-diesel fuel emulsions, we previously showed that diffusion of surfactant to a highly curved, micrometer-sized droplet in flow is faster than that to a planar interface, resulting in the transport shifting toward being limited by kinetics rather than diffusion. 30 These observations lead to the conclusion that coalescence behavior in emulsions, which contain micrometer-scale droplets, could be influenced not only by the surfactant concentration but also by the surfactant transport to and at the interface. Therefore, there is clearly a pressing need to conduct more controlled single-droplet coalescence experiments in the presence of surfactants on length and time scales relevant to real emulsions with microscale dispersed droplets.…”

Single bubble and drop techniques for characterizing foams and emulsions

“…Microfluidic methods, which require the use of a convective bulk flow, have also been developed to measure interfacial stress at micron-scale interfaces. Additional details on this technique can be found in references [164,165].…”

Qualidade de Combustíveis e as Novas Políticas Ambientais

“…Logo, um aditivo eficaz deve apresentar uma boa estabilidade térmica 54 e à oxidação 46,55 , apresentar alto potencial energético para favorecimento da queima nos motores, miscibilidade adequada com os combustíveis e em alguns casos, ser um agente contra microrganismos que possam proliferar nos tanques dos veículos 56 . Atividades adicionais podem estar contidas em alguns produtos, como por exemplo, poder detergente para limpeza de resíduos no sistema de combustível 57 e ação dispersante de água 58 . De forma geral, o custo adicional associado à utilização dos aditivos é inferior ao custo de reparos mecânicos devido à utilização de combustíveis de má qualidade que possam ter se deteriorado pela ausência de um aditivo adequado [59][60][61] .…”