Fundamental Mechanisms of Fuel/Water Separation, 08-R9489

Principal Investigators
Gary Bessee
Scott Hutzler
Neal Vail
Consultant
Vic Hughes

Inclusive Dates:  07/01/04 – 04/01/06

Background - The purpose of this project is to investigate the effects that fuel additives have on the fuel/water separation process. Typical additives are materials introduced into fuels to improve certain properties, including static dissipation, corrosion resistances, thermal stability, and anti-icing. Traditional fuel test methods do not address the effects of these additives on water-removal devices such as coalescers/separators and strippers. Interfacial tension (IFT) is usually the only measured property when determining if a fuel meets specification. However, previous testing from several SwRI cooperative R&D programs has shown that two fuels having the same interfacial tension can have markedly different water removal results for a given water removal system.

Approach - The objective of this program is to develop a fundamental understanding of the effects of additives on the performance properties of fuels using molecular- and mesoscale modeling combined with experiment.

The specific aims are:

1. Develop computational models of fuel compositions using molecular dynamics and mesoscale simulation. Simulation results will be used to predict the physical properties and phase morphologies of fuel compositions. We will examine the role of critical fluid properties affected by fuel additives and their affect on the performance of the fuel components. The simulation results will be corroborated by physical experiments;
2. Establish fundamental knowledge and predictive tools to significantly expand our capacity to provide fuel formulation development capabilities to our customers. We anticipate developing a database of fuel/additive property and performance data that can be used during fuel formulation development to streamline the additive selection process and provide high-value to our customers. Furthermore, we will develop computational and experimental screening capabilities for determining the performance of new additive/fuel formulations to support new product development requirements of our customers;
3. Establish core "know-how" in the area of fuel/additive formulation development. The knowledge and predictive tools noted above will add considerably to our intellectual property portfolio. We will use these tools to continually develop our fuel formulation development and testing capabilities to meet increasing customer needs; and,
4. Identify areas for continued spinning disk process development.

This research will help to identify some of the fuel/additive/substrate interactions that account for poor fuel/water separation, yet which cannot be completely correlated to interfacial tension. This outcome will be significant in its contribution to the characterization of fuel additives and their effects on the fuel/water separation process. This will lead to alternative additives that are more amenable to current filtration technology and to improve filtration systems that can accommodate a wider variety of additives. The research will also initiate development of new analysis and experimental techniques, and test methodologies that will keep SwRI at the forefront of fuel-based research. This research will further propel SwRI into new and under-explored areas of filtration technology and fuel formulation.

Accomplishments - We have developed mesoscale simulations of model fuel/water systems and shown that we can accurately predict the interfacial tension of these systems when compared to experimental results. We extended these simulations to include model additives experimentally determined to have the most detrimental effects on the fuel/water separation process. Such additives include diethylene glycol monomethylether (DiEGME, an icing inhibitor) and lubricity improver (DCI-4A). Figure 1 shows a snapshot from a representative simulation for the DiEGME/fuel/water system. Initially, the system is a completely random mixture of fuel, water, and additive. As the simulation proceeds, the system phase separates with DiEGME (red ball and yellow sticks) accumulating preferentially in the water phase. This result is consistent with experimental observations of this system and helps to explain the water stabilization effects of DiEGME in fuel systems. As seen in this simulation, the DiEGME is accumulating at the fuel/water interface, indicating it is acting as a surfactant to stabilize the water phase in the fuel.

Based on previous work with the DiEGME/fuel/water system, we extended our work to include interactions of the system with water removal substrates. We found that surface-active additives, such as linoleic acid, will compete with water for binding sites on a hydrophilic surface (Figure 2). This evidence supports the theory that several mechanisms may be at work that ultimately affect fuel/water separation. Because IFT targets only the liquid/liquid interface, it can only provide an accurate assessment with additives that affect the interfacial tension. IFT measurements cannot account for additives that alter the wettability of media surfaces and interfere with coalescence.

Figure 1. Mesoscale simulation - Equilibrium water droplet with DIEGME in a model fuel.	Figure 2. Mesoscale Simulation - Spreading of a water droplet onto a hydrophilic surface. The linoleic acid head group (yellow ball) is seen competing for binding sites on the surface which would interfere with coalescence.

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