Ind. Eng. Chem. Res. 1999, 38, 2497-2502
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Jet Fuel System Icing Inhibitors: Synthesis and Characterization George W. Mushrush,*,†,‡ Erna J. Beal,† Dennis R. Hardy,† Janet M. Hughes,§ and John C. Cummings⊥ Materials Chemistry Branch, Code 6121, Naval Research Laboratory, Washington, D.C. 20375-5342, Chemistry Department, George Mason University, Fairfax, Virginia 22030-4444, Geo-Centers, Fort Washington, Maryland 20749-1340, and The Naval Air Systems Command, Patuxent River, Maryland 20670-1534
There have been no new literature papers dealing with jet fuel deicing additive replacements for the past 20 years. The current fuel system icing inhibitor additives, used by both the military and commercial aviation, are ethylene glycol monomethyl ether and diethylene glycol monomethyl ether. These deicing compounds are toxic at the concentrations that are required for effective deicing. This observation points to an immediate need for nontoxic, inexpensive, and biodegradable deicing compounds. The synthesis of polar sugar derivatives represents a viable alternative to glycol-based additives. The alternative deicing compounds reported in this paper are inexpensive and fuel stable and exhibit icing inhibitor characteristics similar to those of the presently used commercial materials. Introduction The literature of deicing additives for jet fuels is rather sparse. A search of the past 20 years produced no new compounds, either proposed or synthesized for this particular purpose. Those papers that have appeared were related to concentration determination, stability in fuels, and health implications of the current additives.1-5 The fuel system icing inhibitor additives presently used, ethylene glycol monomethyl ether (EGME) and diethylene glycol monomethyl ether (DiEGME), are mandatory in all military aircraft fuels and are optional in worldwide commercial aviation fuels depending on route, flight length, and season. Unfortunately, ethylene glycol based deicing compounds are toxic at the concentrations that are required for effective deicing.5 These additives are leached out of the fuel and into water bottoms. When this water is drained from fuel system sumps, filters, and storage tanks, it contains EGME and/or DiEGME, thus creating a personnel health hazard. Also, glycols exert a high oxygen demand for decomposition, so when they get into the environment, they cause the death of aquatic organisms as dissolved oxygen is depleted. These observations all point to an immediate need for nontoxic, inexpensive, and biodegradable deicing compounds. The approach of our laboratory is to utilize the large U.S. surplus of sugars as the basis for the synthesis of biodegradable deicing compounds. The origins of carbohydrate chemistry can be traced back to antiquity, but their use for deicing purposes is new. At present, the manufacture of carbohydrates is important to many * To whom correspondence should be addressed at Code 6121, Fuels Section, Chemistry Division, Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 203755342. Phone: 202-404-8100. Fax: 202-404-3719. E-mail:
[email protected]. † Naval Research Laboratory. ‡ George Mason University. § Geo-Centers. ⊥ The Naval Air Systems Command. 10.1021/ie9807755
industries including the organic chemical synthesis industry. With such a broad demand, the basic building blocks of carbohydrate chemistry are industrially quite inexpensive and readily available. Potential deicing candidates must satisfy many constraints. They must be soluble in jet fuel, soluble in water, and fuel stable during storage and exhibit iceinhibiting characteristics similar or enhanced to those of currently used deicing compounds. The latter of these constraints, concerning the behavior of deicing compounds in fuels, is being investigated in our laboratory, because there are no readily available software programs to estimate either the physical or colligative properties of middle distillate fuels. Some work has been done involving the determination of partitioning coefficients and equilibrium solubility of icing inhibitors in jet fuel.3 A large number of physicochemical and toxicological properties are prerequisite to a reasonable hazard assessment of a chemical.2 Good quality experimental values for these end points are not available for the majority of the industrial chemicals present in the Toxic Substances Control Act (TSCA) Inventory.1 However, environmental fate and toxicity of chemicals can be estimated using computer models. These predicted values provide the guidance toward synthesizing safer icing inhibitors for this project. In this study, we report on the synthesis, accelerated fuel stability testing, and computer modeling of the environmental fate of these compounds. Experimental Section Synthesis Procedure. A convenient sugar to begin with is mannose because its only reduction product is glycerol. Glycerol can then be used as the starting reactant for all of the following reactions. The general reaction procedure followed for the synthesis of the glycerol acetals and ketals employed in this study was that reported for the synthesis of 2,2-dimethyl-1,3dioxolane-4-methanol, in Scheme 1.6 The procedure was modified for the synthesis of the formaldehyde, and acetaldehyde, adducts. Acetone (232 g, 4.5 mol), acetaldehyde (197 g, 4.5 mol), or formaldehyde (135 g, 4.5 mol)
This article not subject to U.S. Copyright. Published 1999 by the American Chemical Society Published on Web 04/30/1999
2498 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 Scheme 1
was added to glycerol (100 g, 1.1 mol) in a toluene solvent (300 mL), containing 3.0 g of p-toluenesulfonic acid and 255 g of 5A molecular sieves all in a 2000 mL two-necked, round-bottomed flask fitted with a mechanical stirrer and a condenser. A freezing mixture of ethylene glycol and water at -25.0 °C was circulated through the condenser. The stirred reaction mixture was heated under gentle reflux for 33 h using a heating mantle. After reflux, the condenser was disconnected and excess acetaldehyde was allowed to evaporate. The acidic reaction mixture was neutralized with 3.0 g of sodium acetate. The molecular sieves were separated by vacuum filtration using a Bu¨chner funnel. The resulting liquid was distilled under vacuum. The colorless organic product distilling at 80-82 °C/10 mm was collected for the acetone derivative to give a yield of 88%; for the acetaldehyde derivative the product distilling at 85-90 °C/1 mm was collected to give a yield of 80%; and for the formaldehyde derivative the product distilling at 95-96 °C/10 mm was collected.7 The products once purified were stored under refrigeration until use. Fuels. The fuels used in this study, a military JP-8 and a commercial Jet-A from Paige Airways at Dulles International Airport in Fairfax, VA, were fuels that were found to be stable on storage and to have no deleterious reactions such as peroxide formation or sediment formation. These fuels have been used in other studies and have been found to be chemically inactive. Storage Stability Procedure. Solutions of 0.15 vol % of deicing compound were prepared in both stable military JP-8 and commercial Jet-A fuel. The military requirement for deicing additive concentration is 0.03 vol % for the Navy and 0.07 vol % for the Army/Air Force/NATO usage. A concentration double the Army value was chosen because an additive passing the stability test at this high concentration would ensure that at the lower concentration there could not be a problem that had not been anticipated. These mixtures were then tested for fuel instability and incompatibility reactions. They were tested by a modified ASTM method D5304-94.8 In brief this method can be described as 100 mL samples (0.15 vol %) contained in 125 mL brown borosilicate glass bottles subjected to a 16 h, 90 °C
time-temperature regimen at 50 psig overpressure of air. After the stress period, the samples were cooled to room temperature. These samples were tested for fuel sedimentation by a gravimetric technique and peroxidation by a modified procedure based on the ASTM D 3703-92 procedure.9 Successfully passing this accelerated testing regimen is a valid indicator that these compounds do not induce instability in an otherwise stable fuel. Freezing Point Rig Procedure. The freezing point studies were performed in the U.S. Navy Fuel System Icing Simulator at the Naval Air Warfare Center Aircraft Division, Trenton, NJ. This test rig is a smallscale, recirculating simulator which can be used to test the effectiveness of Fuel System Icing Inhibitor additives at varying concentrations and varying amounts of total water present. Solutions were prepared that consisted of 0.01-0.20 vol %. They were then tested for freezing point lowering. The solutions were gradually cooled to -37 °C (military requirement) and the solutions recycled through the apparatus. A deicing compound was considered ineffective if it reached a 35 psi differential across a 30 µm filter in 6 h or less. Extensive testing has shown that if the deicing additive is enough to reach 6 h without the differential pressure reaching 35 psi, the test can be run virtually nonstop without the filter differential pressure ever reaching 35 psi. Computational Methods. To estimate environmental fate and certain physical properties, a suite of programs developed by Syracuse Research Corp. (SRC) was used.10 Well-established computational methods are used in these programs. Dermal permeability and dose per event were calculated using DERMAL.11 DERMAL requires the input of a structure in SMILES notation, the event duration, and water concentration. DERMAL uses the estimated methods outlined in the EPA document “Dermal Exposure Assessment: Principles and Applications”. The values for Kp (dermal permeability) were taken from the equation best representing the class of compounds being studied.12 The values for dose per event were calculated by DERMAL using Flick’s first law, with concentration set at 100 mg/cm3 per 15 min event. DERMAL was used to calculate the value for log(p)(Kow), the n-octanol/water partition coefficient. Vapor pressure was calculated using the program MPBP, which uses a boiling point estimation to arrive at an estimate of vapor pressure using three different methods.10a,13 Water solubility was predicted using the program WSKOW, which uses EPA established methods for its calculations.10b Henry’s law constant was estimated using the bond contribution method from the program HENRY.10c The OH radical rate constant was predicted using the program AOP, which also calculated the atmospheric half-life of each compound based upon 12 h of daylight, with a concentration of 1.5 × 106 OH radicals/cm3.10d The soil adsorption coefficient, Koc, was estimated using the program KOC, which uses methods developed by SRC.10d,e Aquatic half-lives were calculated using the previously described programs in combination with SRC’s Estimation Program Interface.10f The parameters were based upon a model lake depth of 1 m, a wind velocity of 3 m/s, and a current velocity of 1 m/s. All predicted values are at 25 °C. ASTER, a risk assessment tool developed at the United States Environmental Protection Agency, was used to calculate three ecotoxicological end points.14 These end points included the biodegradation half-life
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2499
Figure 1. Time vs temperature relationships for fuel system icing inhibitor compounds 2,2-dioxolane-4-methanol and 2-methyl-1,3dioxolane-4-methanol and the currently used additive diethylene glycol monomethyl ether and with dipropylene glycol.
in days, acute toxicity (LC50) concentration in µg/L for Pimephales promelas, and bioconcentration factor (BCF) in P. promelas. Results and Discussion The reaction products of aldehydes, i.e., acetaldehyde and formaldehyde, and ketones, i.e., acetone, with glycerol have been known for more than 100 years. It was not until the late 1950s that a systematic study of the mechanism of these reactions was undertaken.15 These compounds were regarded as intermediates in synthetic procedures, and little interest was expressed in them. These compounds are much simpler chemically than the carbohydrates and carbohydrate derivatives so they were the subject of this initial investigation. The present paper is intended to complement the already abundant literature with specific new uses for these compounds. Acetal and ketal formation is catalyzed by either mineral acids (HCl) or Lewis acids (p-toluenesulfonic acid). The overall reactions for the synthesis of the compounds of this study are presented in Scheme 1. The hemiacetals or hemiketals are not usually isolated because these compounds are not very stable and their isolation would not be significant. The reactions reported in Scheme 1 are general reactions that could be employed for any glycol that had at least one diglycol functional group. The compounds were subjected to testing for deicing characteristics and compared to EGME, DiEGME, and dipropylene glycol. Dipropylene glycol was included because the Federal Aviation Administration has suggested it as a replacement for the ethylene-based deicers. The freezing point tests were conducted in the 1 gal Fuel System Icing Simulator test rig. The results are shown in Figures 1 and 2. The data illustrated in Figures 1 and 2 showed that both 1,3-dioxolane-4methanol and 2-methyl-1,3-dioxolane-4-methanol were
effective deicers and closely paralleled the behavior of EGME and DiEGME.3,4 Figure 1 shows that these compounds showed time vs temperature dependence similar to that of the presently used materials. This graph answers two questions: (1) What is the lowest fuel temperature that can be achieved for a given additive concentration before the water in the fuel freezes? (2) Is the test rig operating properly? Extensive statistical analyses over several years of tests run with EGME and DiEGME in JP-5 were used to generate a baseline time/temperature repeatability curve against which subsequent operation of the rig could be compared to ensure proper interpretation of the results with these new additives. Figure 1 definitely shows that the new compounds achieve the required temperature -37 °C and mimic the behavior of DiEGME. Figure 2 answers the question, how long will it take the water in the fuel to freeze and plug the filter for a given concentration of FSII additive? It also shows the time it takes to reach a 35 psi differential pressure across the 30 µm filter for a given FSII concentration. The upper time limit for the test is fixed at 6 h (360 min). If the differential pressure of 35 psi is not encountered prior to this time, the test is stopped. Testing protocol over many years with this rig has shown that if the additive concentration is enough to reach 6 h without the differential pressure reaching 35 psi, the test can be run virtually nonstop without the filter differential pressure ever reaching 35 psi. The compounds were tested for fuel instability and incompatibility reactions. They were tested for storage stability by ASTM method D5304-94 in JP-8.8 The JP-8 fuel containing the sugar-derived compounds was subjected to a 16 h, 90 °C time-temperature regimen at 100 psig overpressure of oxygen.16 All of the compounds were tested for fuel sedimentation and for peroxidation. None of the added acetal or ketal compounds formed
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Figure 2. Time vs volume percent relationships for fuel system icing inhibitor compounds 2,2-dioxolane-4-methanol and 2-methyl-1,3dioxolane-4-methanol and the currently used additive diethylene glycol monomethyl ether and with dipropylene glycol. Table 1. Estimated Values for Current Additives Ethylene Glycol Monomethyl Ether and Diethylene Glycol Monomethyl Ether ethylene glycol monomethyl ether (EGME) dermal permeability Kp, cm/h dermal dose per event (concentration of 100 mg/cm3 for 0.25 h), mg/cm2 log Kow (lipophilicity) vapor pressure, mmHg water solubility, mg/L Henry’s law constant, atm x m3/mol OH rate constant, cm3/molecule s atmospheric half-life, h soil adsorption coefficient Koc volatilization from model river (half-life), years volatilization from model lake (half-life), years biological oxygen demand (BOD) (half-life), days LC50 for P. promelas, µg/L bioconcentration factor (BCF) for P. promelas
any measurable solids (
