Article pubs.acs.org/EF
Equilibrium Partitioning of Di-ethylene Glycol Monomethyl Ether (DiEGME) between Fuel and Aqueous Phases at Sub-Ambient Temperatures Zachary J. West,† Linda M. Shafer,† Richard C. Striebich,† Steven Zabarnick,† Charles Delaney,‡ Donald Phelps,§ and Matthew J. DeWitt*,† †
University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469, United States Encore Support Systems, 303 Clarence Tinker Drive, San Antonio, Texas 78226, United States § Air Force Research Laboratory, Fuels and Energy Branch, Wright-Patterson Air Force Base (WFAFB), Ohio 45433, United States ‡
ABSTRACT: Improved understanding of the effect of temperature and concentration on the equilibrium partitioning of Fuel System Icing Inhibitor (FSII) additive between fuel and aqueous phases can assist in identifying required dose concentrations for safe aircraft operability. A novel experimental system was designed and used to quantify the equilibrium partitioning of the currently approved FSII, di-ethylene glycol monomethyl ether (DiEGME), under conditions relevant to actual aircraft fuel system operation. This included temperatures from ambient to −47 °C, total water contents from 130 to 560 ppmV, and initial FSII concentrations from 100 to 1500 ppmV. The partitioning of DiEGME was a strong function of temperature, exhibiting nonideal solution behavior. For a constant temperature, the resulting phase partitioning was independent of initial FSII and total water concentrations, with a single equilibrium correlation established. FSII partitioning into the aqueous phase increased with both decreasing temperature and initial FSII dose concentration in the fuel. The overall behavior was attributed to hydrophilic interactions between the glycol ether and water, which become more favored at lower temperatures and concentrations. The behavior is consistent with that expected based on the effect of temperature and concentration on the corresponding FSII activity coefficients in each phase, and has previously been observed for analogous glycol ethers. Based on the partitioning behavior, very low concentrations of FSII are expected to be sufficient to prevent water solidification to temperatures below the specification freeze point of the fuel.
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INTRODUCTION Fuel system icing inhibitor (FSII) is one of three additives currently required in U.S. military aviation fuels JP-5 and JP-8, per specifications MIL-DTL-5624V1 and MIL-DTL-83133H,2 respectively. The primary function of FSII is to prevent water, present in fuel systems at parts per million levels, from solidifying and inhibiting aircraft operation. The requirement for FSII was added in 1961 following several reported incidents that attributed fuel system malfunctions and aircraft losses to ice formation in the fuel system; an extensive research and development program was performed to evaluate and approve a FSII additive.3−5 The FSII additive currently approved for use is di-ethylene glycol monomethyl ether (DiEGME) (CAS No. 111-77-3); the structure is shown in Figure 1. DiEGME has
fuel transport and storage, thus ensuring the minimum requirement for aircraft use. Commercial aviation fuels Jet A and Jet A-1 do not require FSII, but the additive is permitted for use with a fuel procurement range of 0.10−0.15 vol % (per ASTM D1655-13a8). ASTM is currently considering reduction of the minimum procurement limit to 0.07 vol %, which will be consistent with the USAF requirement. There are many motivating factors for determining the lowest FSII dose concentration which can provide the desired anti-icing efficacy during operation. These range from logistical and economic issues to application-based concerns. A lower FSII requirement would significantly reduce the associated logistical footprint and procurement issues of the additive, rendering a considerable cost savings, especially for operation outside of the United States. In addition, water bottoms containing DiEGME are considered hazardous waste and have disposal- and toxicity-related concerns. DiEGME has been implicated in the peeling of fuel tank topcoat material in the ullage space of integral wing fuel tanks in the B-52 and other military aircraft; a lower concentration may reduce the frequency of failure occurrences.9,10 From an application perspective, the need for FSII may be reduced or eliminated due to more stringent maintenance practices (e.g., improved
Figure 1. Structure of DiEGME.
both hydrophilic and hydrophobic functionalities and is soluble, to varying degrees, in both fuel and water. FSII is required at the highest concentration of the military fuel additives with procurement ranges of 0.07−0.10 vol % (JP-8) and 0.10−0.15 vol % (JP-5), and on-aircraft minimum use limits of 0.03 vol % (300 ppm) for JP-5 (per NAVAIR 00-80T-1096) and 0.04 vol % for JP-8 (per USAF T.O. 42B-1-17). The higher procurement range allows for possible FSII loss via water extraction during © 2014 American Chemical Society
Received: April 21, 2014 Revised: June 20, 2014 Published: June 25, 2014 4501
dx.doi.org/10.1021/ef500900p | Energy Fuels 2014, 28, 4501−4510
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Article
%). The freeze point dependence shown in Figure 2 indicates that a single eutectic point may exist at temperatures below −70 °C; however, it was not possible to obtain quantitative freeze point measurements within this concentration range because of the reduced amount of latent heat released. The inability to obtain quantitative freeze points could also potentially be due to a larger number of molecular conformations formed at higher glycol ether concentrations, which can result in multiple types of solid phases. Similar behavior has previously been reported for the solid−liquid phase transition of ethylene glycol/water mixtures, where different phase formation could be promoted, depending on the cooling methodology used.17 For the ethylene glycol system, a stable hydrate phase could be produced by seeding crystals for nucleation and growth resulting in a phase diagram with two eutectic points. The DiEGME/water system is more complex because of the varying types of oxygen bonding within the molecule and may allow formation of multiple crystal structures. Regardless of whether the DiEGME/water system has a single or multiple eutectic points, the respective freeze points within the 60−100 vol % range are most likely well below −50 °C. Since the specification fuel freeze points for JP-5 and JP-8 are −46 and −47 °C, respectively, there should be minimal concerns with ice formation once the freeze point of the FSII/water phase is below these values. In fact, this would provide for safe operability, regardless of the aircraft platform or mission profile. Since FSII is added to the fuel, and must subsequently partition into any free water present, determination of the final phase equilibrium as a function of FSII/water concentration and temperature is highly relevant. This would not only provide a measure of the “potential protection” during flight (via freeze point depression) and guidance for a minimum required additive dose concentration, it would also provide guidance of potential losses during fuel procurement and transfer.18,19 Partition Coefficients of FSII in Fuel/Water Systems. The equilibrium ratio of FSII between fuel and aqueous phases provides guidance regarding the potential anti-icing efficacy with varying FSII dosage concentrations. For aviation applications, this ratio has previously been estimated using the Partition Coefficient (PC), defined as
sump draining), alternate system designs for newer aircraft, and hardware modifications made to legacy aircraft (e.g., water scavenge rakes). In addition, FSII is not required during commercial aircraft flight, indicating the potential for safe operation without the additive. However, there are significant differences in the operation cycle of commercial and military aircraft. Extensive studies have been performed to determine the minimum required DiEGME concentration to prevent blockage of close tolerance flow paths due to ice formation. Most notably, the U.S. Navy performed the majority of this testing using the Fuel System Icing Simulator (FSIS), which evaluates the plugging of a fine flow restriction (typically, a 30 μm wire mesh filter) by recirculating fuel with a specified total water and FSII content.11,12 These studies demonstrated that the DiEGME concentration can be significantly lower than the 0.07 vol % procurement limit and still prevent solidification of water at very low temperatures (50 vol %). 4502
dx.doi.org/10.1021/ef500900p | Energy Fuels 2014, 28, 4501−4510
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studied. Orrell and co-workers21 performed a more-detailed study investigating the PC of EGME as a function of temperature and concentration for varying fuel compositions. These studies were primarily conducted with high water concentrations (2500 ppmV) for temperatures from −50 °C to +24 °C. The [DiEGME]aq was quantified while the fuel phase concentration was calculated by mass balance based on initial concentrations. The PC was found to increase significantly as the temperature was reduced at constant FSII/water concentrations. For example, the PC values for testing with initial concentrations of 0.60% EGME and 0.25% water (FSII/water ratio of 2.4) were 119 (24 °C), 172 (−5 °C), 197 (−30 °C), and 353 (−50 °C). However, the PC value was found to decrease as the EGME/water ratio increased with a constant total water or EGME concentration. Limited studies have been performed to estimate the PC of DiEGME in aviation fuels, and all have been performed at ambient temperature. PC measurements were conducted for EGME and DiEGME in JP-4 and JP-5 (400 mL fuel), using consecutive extractions of the FSII from the fuel using large quantities of water.22 The first extract (10 mL water) was analyzed to quantify [DiEGME]aq while the second extract (40 mL water) was used to determine [DiEGME]fuel. Reported values from the study are shown in Table 1. DiEGME was
current applications is limited due to the high water concentrations used. Therefore, this study was performed to investigate the effect of the pertinent variables on FSII partitioning using concentration and temperature ranges relevant to current aircraft fuel system operation.
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A novel experimental system and testing methodology were developed to quantify the PC of DiEGME between fuel and water under aircraft relevant conditions. The variables of interest included the temperature and the total concentrations of DiEGME and water. The temperature range of interest is from ambient conditions to the minimum that bulk fuel could experience during flight, −47 °C, per the JP-8 and Jet A-1 fuel freeze point specifications. The DiEGME concentration range is bound by a minimum level of interest, 0.01% by volume, to the current maximum procurement limit (0.15 vol % in JP-5). The expected total water content is difficult to specify directly, but recent estimations and measurements of the total water content expected on aircraft indicate that low quantities of water (
