Unleaded aviation gasoline

Details Unleaded aviation gasoline Unleaded aviation gasoline

1993-11-08

2001-05-29

Johnson, Jerry D. (Department: 1764)

Fuel and related compositions

Liquid fuels

Aluminum or heavy metal, other than lead, containing

C044S449000

Reexamination Certificate

active

06238446

ABSTRACT:

This invention relates to unleaded aviation gasoline compositions. More particularly, this invention provides unleaded high octane aviation gasoline compositions which can achieve performance levels comparable to, if not better than, present-day aviation gasolines. Additionally, this invention accomplishes this important advantage on an economical basis, while at the same time conserving worldwide petroleum resources.
While leaded aviation gasolines have performed wonderfully well in actual service for many years, pressures are being applied to eliminate use of leaded aviation gasoline. If these efforts succeed, the refining industry will be faced with the problem of trying to provide unleaded aviation gasoline that performs as well as leaded aviation gasoline and that does not exceed the economic constraints of the marketplace. In fact, a scientific debate exists as whether it is even possible to produce an unleaded aviation gasoline comparable to the so-called 100/130 low-lead aviation gasoline now in widespread use in the United States. While petroleum refiners generally believe this to be possible, they also believe that the fuel will be very expensive.
When attempting to eliminate use of alkyllead antiknock compounds in aviation gasoline base fuels, it is essential to provide aviation fuel compositions which not only have the requisite octane quality but additionally have the requisite heat of combustion, as this is a measure of the distance an aircraft can fly before refueling. Accordingly, this invention has as its principal object the provision of particular aviation fuel compositions that possess both the necessary octane quality for aviation service and the necessary heat of combustion for aviation service. Another object is to keep the metal content of the fuel composition as low as is consistent with achieving the foregoing objectives.
This invention involves, inter alia, the discovery that it is possible to provide aviation fuels having the necessary heat content (normally expressed in terms of BTU per pound of fuel) and octane quality, by use in forming the fuel of appropriate proportions of aviation alkylate, a gasoline-soluble dialkyl ether octaneblending agent and a cyclopentadienyl manganese tricarbonyl compound. In some cases, it is desirable to also include other suitable gasoline hydrocarbon components in the finished aviation fuel composition, such as isopentane, suitable aromatic gasoline hydrocarbons, light hydrocracked gasoline fractions, and/or C
5-6
gasoline isomerate in order to ensure that the composition possesses the requisite combination of properties. It will be appreciated therefore that the present invention is an economical way of providing unleaded aviation gasolines having the requisite octane quality and heat of combustion to satisfy aviation engine requirements.
In accordance with this invention, there is provided an unleaded aviation gasoline composition which comprises:
(a) from 85 to 92 volume percent of aviation alkylate;
(b) from 4 to 10 volume percent (preferably about 4 to about 8 volume percent), of at least one ether selected from methyl tertiary-butyl ether, ethyl tertiary-butyl ether, methyl tertiary-amyl ether, and mixtures of any two or all three of the foregoing ethers;
(c) from zero to 10 volume percent of one or more other hydrocarbons falling in the aviation gasoline boiling range; and
(d) from 0.25 to 0.6 gram, more preferably, in the range of about 0.4 to about 0.6 gram, and most preferably in the range of about 0.4 to about 0.5 gram, of manganese per gallon as one or more cyclopentadienyl manganese tricarbonyl compounds;
wherein the sum of the amounts of (a) and (b), and also of (c) if present, is 100 volume percent; with the proviso that and that (a), (b) and (d), and also (c) if present, are proportioned such that said composition has (i) an ASTM D 2382 heat of combustion of at least 18,000 BTU per pound (and preferably is at least 18,700 BTU per pound), and (ii) a minimum knock value lean rating octane number of 100 as determined by ASTM Test Method D 2700 and wherein motor method octane ratings are converted to aviation ratings in the manner described in ASTM Specification D
910-90.
An ASTM D 2382 heat of combustion value of at least 18,000 BTU per pound is deemed sufficient to provide the range of flight required in actual aircraft service. The preferred minimum value of 18,700 BTU per pound corresponds to the requirement of the present ASTM Specification D 910-90.
A preferred embodiment of this invention is an aviation gasoline composition as above described further characterized by having a minimum supercharged knock value octane number of 130. In other words, the gasoline composition additionally has a minimum performance number reported to the nearest whole number and as determined by ASTM Test Method D 909 of 130. In this connection, a minimum performance number of 130 is equivalent to a knock value determined using isooctane plus 1.28 milliliters of tetraethyllead per gallon.
In particularly preferred embodiments, the aviation alkylate is formed by acid-catalyzed isoparaffin-olefin alkylation wherein the butene fraction of a mixed olefin feedstock is isobutene depleted—i.e., the butene fraction contains, if any, less than 30 percent of isobutene, especially when a hydrofluoric acid alkylation catalyst system is used. Preferably, less than 20% of the butene fraction of the mixed olefin feedstock to the hydrofluoric acid-catalyzed alkylation process is isobutene. Another suitable approach is to use substantially pure isobutene as the olefin feedstock in the hydrofluoric acid-catalyzed alkylation process. Alternatively, the aviation alkylate can be produced by sulfuric acid-catalyzed isoparaffin-olefin alkylation. The aviation alkylates produced in these processes typically are highly branched paraffin hydrocarbons (chiefly in the C
7
to C
9
range) that distill at temperatures in the range of up to 200° C. and have clear octane ratings in the range of 92-96. Alkylation processes for producing aviation alkylate are known in the art of gasoline manufacture and are referred to for example in W. L. Lafferty and R. W. Stokeld,
Adv. Chem. Ser.,
103, 130 (1971); D. Putney,
Advances in Petroleum Chemistry and Refining,
Vol. 2., Interscience Publishers, a division of John Wiley & Sons, Inc., New York, 1959, Chapter 5; R. Dixon and J. Allen, Ibid, Volume 3, Chapter 6; and R. H. Rosenwald,
Encyclopedia of Chemical Technology,
Wiley-Interscience, Third Edition, Volume 2, 1978, pages 52-58. Each of these references is incorporated herein by reference.
In general there are two ways of producing for use in aviation alkylate manufacture, a mixed olefin feedstock depleted in isobutene. One is to remove isobutene from the feedstock by physical separation procedures, such as distillation. The other involves recourse to chemical separation such as by charging the feedstock to a reactor in which the isobutene is selectively reacted with a lower alcohol such as methanol or ethanol to produce methyl tertiary-butyl ether or ethyl tertiary-butyl ether. The remainder of the feedstock from which isobutene has been removed is recovered for use in producing the aviation alkylate. For further details concerning the processing useful in selectively reacting isobutene with a lower alcohol to form the ether, reference may be had, for example, to U.S. Pat. Nos. 4,528,411; 5,243,090; 5,024,679 and E.P. 390,596 A2, each of which is incorporated herein by reference.
As noted above one or more other hydrocarbons falling in the aviation gasoline boiling range can be (but need not be) present in the aviation fuel compositions, provided that the finished fuel blend has the combination of lean value octane quality and heat of combustion content required by this invention. Thus, for example, the fuel blend may contain up to about 10 volume % of aromatic gasoline hydrocarbons, at least a major proportion of which are mononuclear aromatic hydrocarbons such as toluene, xylenes, the mesitylenes, ethyl benzene, etc. Other suitable optional gasol

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