Patent Publication Number: US-2007094918-A1

Title: Composition and method for enhancing the stability of jet fuels

Description:
TECHNICAL FIELD  
      This invention relates to compositions and methods for stabilizing jet fuels and inhibiting deposit formation on jet engine components during combustion of the fuels.  
     BACKGROUND OF THE INVENTION  
      Jet fuel, in addition to its primary use as a propellant is used as a coolant for lubricating systems, electrical systems and environmental control systems in military aircraft. The trends in improving the performance of military aircraft require operation of these engines at ever increasing temperatures. However, jet fuels are known to undergo degradation at high temperatures leading to the formation of gums and particulates that could lead to fuel system failure. This results in an increased demand for jet fuels possessing enhanced stability at higher temperatures.  
      In order to qualify jet fuels for these high temperature operations the Air Force Research Lab (AFRL) has developed standard tests of jet fuel stability referred to as the Quartz Crystal Microbalance (QCM), Hot Liquid Process Simulator (HLPS), and Isothermal Oxidation Test (ICOT) tests. In addition the AFRL would like prospective jet fuels to pass the Water Separometer Index Modified (WISM) test, which is a measure of water retention in treated jet fuel.  
      U.S. Pat. Nos. 5,621,154 and 5,596,130 and a DSTO Aeronautical and Maritime Research Laboratory, Australia, Publication DSTO-TR-1135, April 2001, describe an additive that is a blend of polyalkenylthiophosphonate, a metal deactivator, and an antioxidant. This additive passes the QCM, HLPS, and ICOT tests, but fails to pass the WSIM. Therefore, the additive must be mixed with the fuel on-site at the terminal while fueling the aircraft. This limits the additive&#39;s use at any other point (i.e. storage, transport). It also renders the treated recovered fuel from the aircraft unfit for reuse.  
      Accordingly, there is an ongoing need for additives that enhance high temperature stability of jet fuels (e.g., JP8+100) and meet or exceed the performance standards set forth by the United States Air Force. This requires additives to reduce fouling of Jet Fuels and pass the QCM, HLPS, and ICOT tests. Preferably the additives should not contain undesired phosphorous and also eliminate the problem of water retention by the treated jet fuel during storage and/ or transport.  
     SUMMARY OF THE INVENTION  
      We have discovered that polyalkenyl substituted succinic acid ester and α-olefin maleic anhydride copolymer dispersants in combination with metal deactivators effectively enhance the stability of jet fuels. The additive compositions described herein enhance high temperature stability of jet fuels (e.g., JP8+100) and meet or exceed the performance standards set by the United States Air Force. In addition, certain of the additives also pass the WISM test for water retention in the fuel.  
      Accordingly, in an embodiment, this invention is a composition comprising about 5 to about 95 weight percent of one or more dispersants selected from the group consisting of polyalkenyl substituted succinic acid esters and α-olefin maleic anhydride copolymers and mixtures thereof and about 0.1 to about 25 weight percent of one or more metal deactivators.  
      In another embodiment, this invention is a method of stabilizing jet fuel comprising adding to the fuel an effective stabilizing amount of a composition comprising about 5 to about 95 weight percent of one or more dispersants selected from the group consisting of polyalkenyl substituted succinic acid esters and α-olefin maleic anhydride copolymers and mixtures thereof and about 0.1 to about 25 weight percent of one or more metal deactivators.  
      In another embodiment, this invention is a jet fuel stabilized with an effective amount of a composition comprising about 5 to about 95 weight percent of one or more dispersants selected from the group consisting of polyalkenyl substituted succinic acid esters and α-olefin maleic anhydride copolymers and mixtures thereof and about 0.1 to about 25 weight percent of one or more metal deactivators. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In an embodiment, this invention is a composition comprising about 5 to about 95 weight percent of one or more dispersants selected from the group consisting of polyalkenyl substituted succinic acid esters and α-olefin maleic anhydride copolymers and mixtures thereof and about 0.1 to about 25 weight percent of one or more metal deactivators.  
      Polyalkenyl substituted succinic acid esters may be prepared by reacting polyalkenyl succinic anhydride with monohydric or polyhydric alcohols or aromatic hydroxyl compounds in the presence of catalytic amounts (about 0.1 to about 2 percent) of acids such as p-toluene sulfonic acid or dodecyl benzene sulfonic acid. The molar ratio of polyalkenyl succinic anhydride to alcohol may vary from about 1:1 to about 1:2. The reaction is generally conducted at temperatures between about 100 and about 175° C. The esters may be mono and/ or diesters of succinic acid. The former is partially esterified succinic acid. The esters may also contain free alcoholic or phenolic hydroxyl radicals.  
      Representative monohydric alcohols include, methanol, ethanol, propanol, butanol, benzyl alcohol, and the like as well as other higher molecular weight alcohols. Suitable polyhydric alcohols typically contain about 2 to about 10 hydroxyl groups. Representative polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, glycerol, pentaerythritol, erythritol, sorbitol, mannitol, and the like and sugars such as glucose, galactose and other carbohydrates. Representative aromatic hydroxyl compounds include phenols, napthols, and the like.  
      Polyalkenyl succinic anhydrides useful as starting materials for the polyalkenyl substituted succinic acid esters may be prepared by thermal addition of polyolefins to maleic anhydride at about 140 to about 250° C.  
      Representative polyolefins include polyethylene, polypropylene, polyisobutylene, polybutene and copolymers comprising such olefinic repeating groups. Typical polyolefins have an average molecular weight of about 400 to about 5,000.  
      In an embodiment, the polyalkenyl succinic anhydride is polyisobutenyl succinic anhydride.  
      The polyisobutenyl (PIB) group can have varied composition. The PIB group can contain about 30 to about 250 carbon atoms such that the resulting PIB succinic anhydride has an average molecular weight of about 400 to about 3000. In an embodiment, the PIB group contains about 50 to about 100 carbon atoms and the resulting PIB succinic anhydride has an average molecular weight of about 600 to about 1500. In an embodiment, the PIB group contains about 60 to about 90 carbon atoms and the resulting PIB succinic anhydride has an average molecular weight of about 800 to about 1300.  
      Polyalkenyl succinic anhydrides are commercially available, for example from Chevron Oronite Company LLC, Oronite Additive Division, Belle Chasse, La., Infmeum, Linden, N.J. and Ethyl Corporation, Richmond, Va..  
      The composition of this invention also comprises one or more ac-olefin-maleic anhydride copolymers in which the anhydride moieties along the polymer backbone are substantially intact and not converted into a di-acid or other anhydride reaction product. The copolymers are composed of one or more α-olefins having from about 10 to about 36 carbon atoms. In an embodiment, the α-olefins have from about 24 to about 28 carbon atoms. Representative α-olefins include 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, 1-docosene, 1-tetracosene, 1-heptacosene, 1-triacontene, 1-hexatriacontene, and the like.  
      In an embodiment, the α-olefin-maleic anhydride copolymers have a weight ratio of α-alpha olefins to maleic anhydride of about 1:1 to about 5:1.  
      In an embodiment, the α-olefin-maleic anhydride copolymers have an average molecular weight of about 5,000 to about 100,000.  
      In an embodiment, the α-olefin-maleic anhydride copolymers have a weight ratio of α-olefins to maleic anhydride of from about 1:1 to about 4:1 and a weight average molecular weight of from about 5,000 to about 20,000.  
      The α-olefin/maleic anhydride copolymers may be prepared by a neat free radical polymerization of the maleic anhydride and the α-olefin as described in U.S. Pat. No. 5,232,963, incorporated by reference. The copolymerization can be initiated by any free radical producing compound including peroxides, azo, and like initiators well known in the art. A preferred initiator is t-butyl perbenzoate. Typically, the initiator concentration is between about 0.001 to about 0.20 moles initiator per mole of maleic anhydride monomer, preferably 0.05 to about 0.10 moles initiator per mole of anhydride.  
      The polymerization may be conducted at a temperature of about 20° C. to about 200° C., preferably about 125° C. to about 175° C. The polymerization pressure may vary from under a partial vacuum up to several thousand psi. Atmospheric pressure to about 100 psi is preferred.  
      Suitable reaction time is usually sufficient time to substantially completely react the available maleic anhydride. Reaction time is typically from about 1 to about 24 hours.  
      The reaction medium should be a liquid at the temperature and pressure of the copolymerization reaction. Suitable solvents which can optionally be employed include liquid saturated and aromatic hydrocarbons having from about 6 to about 20 carbon atoms, halogenated hydrocarbons having from about 1 to about 5 carbon atoms and ketones having from about 3 to about 6 carbon atoms. A neat polymerization reaction conducted in the heated α-olefin comonomer is preferred. Otherwise, it is desirable that a separate reaction solvent be compatible with the end use hydrocarbon stream.  
      As noted above, the composition of this invention further comprises about 0.1 to about 25 weight percent of one or more metal deactivators.  
      In an embodiment, the metal deactivators are selected from N,N′-Salicylidene-1,2-propanediamine, N,N-Disubstituted aminomethyl-1,2,4-triazoles, alkyl phenol-monoethylene diamine and polyethylene polyamine-formaldehyde resin polymers, and mixtures thereof. In an embodiment, the metal deactivator is N,N′-Salicylidene-1,2-propanediamine.  
      In an embodiment, the composition comprises about 10 to about 60 weight percent of one or more dispersants selected from the group consisting of polyalkenyl substituted succinic acid esters and α-olefin maleic anhydride copolymers and mixtures thereof and about 1.0 to about 20 weight percent of one or more metal deactivators.  
      In an embodiment, the composition comprises about 10 to about 40 weight percent of one or more dispersants selected from the group consisting of polyalkenyl substituted succinic acid esters and α-olefin maleic anhydride copolymers and mixtures thereof and about 5.0 to about 10 weight percent of one or more metal deactivators.  
      The composition may further comprise about 5 to about 60 weight percent of one or more antioxidants. Suitable antioxidants may be phenol and amine based. Representative phenolic antioxidants include hindered phenols such as butylated hydroxytoluene, 2,6-di-tert-butyl phenol, and the like. Representative amine based antioxidants include substituted 1,4-diamino benzenes such as N,N′-di-butyl phenlylenediamine, N,N′-di-methylpentyl phenylenediamine, N,N′-di-ethylhexyl phenylenediamine, N-aryl-N′-alkyl phenylenediamines, N,N′-di-aryl phenylene diamines, and the like.  
      In an embodiment, the composition comprises about 10 to about 50 weight percent of antioxidants. In an embodiment, the composition comprises about 10 to about 25 weight percent of antioxidants.  
      The dispersants, metal deactivators and optional antioxidants may be formulated in a variety of hydrocarbon solvents and hydrocarbon solvent mixtures. Representative hydrocarbon solvents include paraffin and aromatic solvents such as jet fuel, diesel, gasoline, heavy aromatic naphtha, gas oil, and the like. In an embodiment, the dispersants, metal deactivators and optional antioxidants are formulated in heavy aromatic naphtha (HAN).  
      The composition of this invention is added to the jet fuel in an amount sufficient to stabilize the fuel and prevent fouling and deposit formation under jet engine operating conditions. Typically, about 0.1 to about 5,000 parts per million (ppm) of the composition is added to the fuel. In an embodiment, about 1 to about 1,000 ppm of the composition is added to the fuel.  
      Suitable jet fuels are generally those hydrocarbon fuels having boiling ranges within the limits of about 150° F. to about 600° F. or higher and are designated by such terms as JP-4, JP-5, JP-7, JP-8, JP-8+100, Jet A and Jet A-1. JP-4 and JP-5 are fuels defined by U.S. Military Specification MIL-T-5624-N, while JP-8 is defined by U.S. Military Specification MIL-T83133D. Jet A, Jet A-1 and Jet B are defined by ASTM specification D1655. These temperatures are often what the turbine combustion fuel oil is subjected to prior to combustion. The fuels may also contain additives which are required to make the fuel oils conform to various specifications including antioxidants, metal deactivators, static dissipators, corrosion inhibitors, fuel system icing inhibitors, and the like.  
      The foregoing may be better understood by reference to the following examples, which are presented for purposes of illustration and are not intended to limit the scope of this invention.  
     EXAMPLE 1  
     Preparation of Polyisobutenyl Pentaerythritol Succinate  
      A mixture of polyisobutenyl succinic anhydride (Mw 1000, 50.3 g, Oronite OLOA-15500, Oronite Company LLC, Oronite Additive Division, Belle Chasse, La.), pentaerythritol (5.1 g) containing catalytic amount (0.5 g) of p-TSA is heated to 200° C. with stirring under a constant stream of N 2  gas. The esterification is monitored by disappearance of anhydride peak and appearance of ester peak in the IR. The water formed in the reaction is collected in a Dean Stark. The reaction went to completion after 90 minutes at this temperature. The reaction mixture is cooled to 150° C. and heavy aromatic naphtha (124 g) is added to dilute the product. The unreacted pentaerythritol is removed by filtration through celite. The clear amber colored product is collected for blending with the metal deactivator to make the inhibitor for fuel stability testing.  
     EXAMPLE 2  
     Preparation of Representative Additive Compositions  
      Representative additive compositions are prepared by blending a polyalkenyl substituted succinic acid esters or α-olefin maleic anhydride copolymers and a metal deactivator into HAN in the desired amount. Representative additive formulations are shown in Table 1. In Table 1, Dispersant A is polyisobutenyl pentaerythritol succinate prepared according to the method of Example 1. Dispersant B is an α-olefin maleic anhydride copolymer having a molecular weight of 10,000-20,000) prepared from 1-octacocene according as described in U.S. Pat. No. 5,232,963. Dispersant C is a commercially available polyalkenyl substituted succinic acid ester (Lubrizol 5948, available from Lubrizol Lubricant Additives, Wickliffe, Ohio). The metal deactivator D is N,N′-Salicylidene-1,2-propanediamine (KMD-80, available from Ferro Corporation, Hammond, Ind.).  
               TABLE 1                          Representative Additive Compositions                                         Disper-   Disper-   Disper-   Metal               sant A   sant B   sant C   Deactivator D   HAN                                                 Additive 1   25%           5%   70%       Additive 2       25%       5%   70%       Additive 3           10%   5%   85%                  
 
     EXAMPLE 3  
     Quartz Crystal Microbalance (QCM) Testing  
      The QCM testing is conducted in a Parr Bomb with 90 ml of JP-8 fuel. The fuel is saturated by bubbling air through it for 30 minutes. The sample is then heated in a closed environment while monitoring the frequency of gold plated quartz 5 MHz crystal (Matex 149211-11) for greater than 15 hrs at 140° C. The deposition rate of foulant formed on the crystal is measured in μg cm −2 . The data from the blank and treated runs is presented in Table 2.  
               TABLE 2                          QCM Testing of Unstabilized and Stabilized Fuel                                     Base   Additive   Surface Depos-   Pass/   % Pro-           Fuel   Conc. (ppm)   it (μg cm −2 )   Fail   tection   Comments                                             JP-8   0   17.24   —   —   Neat base fuel       JP-8   256   1.47   Pass   92   Neat base fuel +                           Additive 3       JP-8   256   2.57   Pass   85   Neat base fuel +                           Additive 2       JP-8   256   2.76   Pass   84   Neat base fuel +                           Additive 1                 Criteria: Deposition &lt;1 μg cm −2  or at least as good as approved additive             
 
      The QCM data collected from these screenings clearly demonstrates a significant reduction in the amount of deposit formed from treated samples on gold crystal. These numbers are within the limits for all three additives prescribed by AFRL for an additive to have passed this test.  
     EXAMPLE 4  
     Hot Liquid Process Simulator (HLPS) Testing  
      The performance of the three additives is then evaluated with an Alcor Hot Liquid Process Simulator. The tests are conducted under AT mode with a rod temperature of 635° F and a flow rate of 3-ml/ min. The differential pressure is measured across a 17 μm in-line filter over a duration of 300 minutes. The JP-8 fuel treated with 256 ppm of each of the three additives gave nearly 100% protection relative to the blank. The data are shown in Table 3.  
               TABLE 3                          Hot Liquid Process Simulation Data                                 Fuel   ΔP   % Protection                       JP-8   300 psi in 90 min   —           JP-8 + Additive 3   0 psi in 300 min   100           JP-8 + Additive 2   0 psi in 300 min   100           JP-8 + Additive 1   0 psi in 300 min   100                         Criteria: ΔP = 0 psi in 300 minutes             
 
     EXAMPLE 5  
     Isothermal Oxidation Test (ICOT)  
      The jet fuel (20 ml) is continually heated at 180° C. while bubbling air at 1.3 L/min for 5 hours. The sample is allowed to stand overnight and the sediment is collected by filtration through a 0.2 mm Whatman nucleopore membrane filter and dried. The results are reported in mg deposit/L of the fuel. The data on the performance of three Nalco&#39;s additives in thermal oxidation test relative to the blank is presented in Table 4.  
               TABLE 4                          Isothermal Oxidation Test Data                                     Additive   Dosage (ppm)   Deposit (mg/L)   % Reduction                                                 Blank   0   431.6               JP-8 +   256   21.6   95.0           Additive 3           JP-8 +   256   7.5   98.3           Additive 2           JP-8 +   256   23.3   94.6           Additive 1                         Criteria: &lt;10 mg L −1              
 
     EXAMPLE 6  
     Water Separometer Index Modified (WSIM) Test  
      The test is conducted by creating a water-depolarized fuel emulsion using a high-speed mixer. The emulsion is then expelled at a programmed rate through a standard fiberglass coalescer and the effluent is analyzed for uncoalesced water by light transmittance measurements. The results are reported on a 0-100 scale. High ratings indicate that water is easily coalesced, implying that the additive does not affect the fuel. The test data is presented in Table 5.  
               TABLE 5                          WSIM Test Data                             Fuel   WSIM (% Transmittance)                                         JP-8 (Depolarized)   91           JP-8 + Additive 3   59           JP-8 + Additive 2   28           JP-8 + Additive 1   86                      
 
      The results from Table 5 data show that Additive 1 passed the WSIM test.  
      Moreover, the data obtained from the testing described above indicates that additives according to this invention passed the initial testing protocol established by the Air Force Research Lab (AFRL) for high temperature jet fuels.  
      Changes can be made in the composition, operation, and arrangement of the method of the invention described herein without departing from the concept and scope of the invention as defined in the claims.