Fundamental Mechanisms of Fuel and Water Separation, 08-9489

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Principal Investigators
Gary Bessee
Scott Hutzler
Neal Vail
Vic Hughes (Consultant)

Inclusive Dates:  07/01/04 – Current

Background - This project is investigating the effects of fuel additives on the fuel and 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 and 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 research and development 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. The team 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 SwRI's capacity to provide fuel formulation development capabilities to its clients. We anticipate developing a database of fuel and 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 clients. Furthermore, we will develop computational and experimental screening capabilities for determining the performance of new additive and fuel formulations to support new product development requirements of our clients;
  3. Establish core "know-how" in the area of fuel and additive formulation development. The knowledge and predictive tools noted above will add considerably to SwRI's intellectual property portfolio. We will use these tools to continually develop our fuel formulation development and testing capabilities to meet increasing client needs;
  4. Identify areas for continued spinning disk process development.

This research will help 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 and 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 have shown that we can accurately predict the interfacial tension of these systems when compared to experimental results. Figure 1 shows a representative mesoscale simulation of a model system comprised of benzene and water. The interfacial tension of the system can be estimated from the fully equilibrated system after an interface has formed.

We have extended these simulations to include model additives experimentally determined to have the most disparate effects on the fuel/water separation process. Figure 2 shows a representative simulation for the diethylene glycol monomethylether (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 into water and fuel-rich layers with DiEGME (red ball and sticks) preferentially accumulating 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 and biased toward the water phase, indicating it is acting as a surfactant to stabilize the water phase in the fuel.

We have been studying the DiEGME/fuel/water system in more detail using atomistic modeling to understand the interactions of the system with water removal substrates. This work is still ongoing, but some results have been used to develop mesoscale models to examine bulk fluid interactions with model surfaces. Figure 3 shows a representative simulation of a fuel/water system exposed to an initially hydrophilic surface that is switched to hydrophobic. The water phase initially wets the surface, then forms droplets. 

t = 0 units t = 50 units t = 500 units t = 2,500 units
Figure 1. Mesoscale simulation of a 50/50 benzene/water system. Benzene is represented by green dots and water is represented by blue dots. The interface between water and benzene is represented by blue surfaces.
t = 0 units t = 1,250 units t = 2,500 units t = 8,750 units
Figure 2. Mesoscale simulation of a 75/25 n-HD/DEB system with 25 wt% water and approximately 0.05 wt% DiEGME. The blue surface is the fuel/water interface, and the red surface is the DiEGME/water interface. The red ball and sticks are the DiEGME molecules, and the fuel is represented by the large black areas in the figure. 
t = 0 units t = 10 DPD units t = 50 DPD units t = 250 DPD units
Figure 3. Mesoscale simulation of a n-hexadecane/water system with a hydrophilic wall. The wall is located at the planes on the left and right of the figure. Water is represented by blue dots, and HD is represented by green dots.

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