US3157989A - High energy fuel and method of minimizing boron oxide deposits - Google Patents

Description

NOV- 24, 1964 w. H. scHEcHTl-:R ETAL 3,157,939

HIGH ENERGY FUEL. AND METHOD 0F MINIMIZING BORON OXIDE DEPOSITS Filed Feb. 1o, 195s I: f I rin/ofende;

Q /va/cwra@ 4M@ I I :a4/maui@ United States Patent Office Y aisasss Patented Nov. 24, 1964 3,157,939 HIGH ENERGY FUEL AND lvmill F Mitdilt/HZENG BRN @XEDE DEPSHS William H. Schechter, Zeiienople, and David it. Eads,

Allison Park, Pa., assignors to @ailery Chemical Cornpany, Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 10, 1958, Ser. No, 714,427 9 Claims. Cl. 60-35.4)

This invention relates to high energy boron-containing fuel compositions that contain certain compounds which, when the fuel is burned, markedly lower the viscosity of boric oxide, one of the principal products of combustion, and thus aid in removing the boric oxide from the equipment in which the fuel was used.

This application is a continuation-in-part of our copending application, Serial No. 498,736, filed April 1, 1955.

It is well known that when a boron-containing fuel is burned, one of the combustion products is boric oxide, which is a viscous liquid or glass when molten. This oxide must be removed as rapidly as it is formed or it will accumulate until the apparatus in which the fuel is burned is rendered inoperative.

One of the objects of this invention is to provide boroncontaining fuel compositions including an additive which will reduce the viscosity of the boric oxide formed as a result of combustion of the fuel composition so that the boric oxideV can be removed more easily from the equipment after combustion.

Another object is to provide high energy fuel compositions containinfy certain alkali metal compounds, and a method of using them, which, when the fuel is burned, will aid in preventing the accumulation of boric oxide.

A further object of this invention is to provide a method of operating7 an engine or motor wherein a boroncontaining fuel composition including an additive is burned and deposits of boric oxide along the exhaust passages are minimized.

In accordance with our invention a boron-containing fuel such, for example, as a liquid boron hydride, a solid boron hydride, a liquid or solid lower alkyl substituted borane, or a liquid or solid hydrocarbon containing elemental boron slurried or dispersed in it, is modified by including therein a small amount of a liquid or solid alkali metal compound or alloy of alkali metals which, upon combustion of ythe resulting mixture, results in a solution of the additive, or its decomposition or combustion product, in molten boric oxide. Consequently, the viscosity of lthe boric oxide is lowered and it is continuously removed from the equipment in which the fuel is burned as rapidly as it is formed.

The alkali compounds or components which can be used in our invention are those which upon combustion in the presence of the fuel result in a solution of the alkali metal additive, or its decomposition or combustion product, in the resulting boric oxide and thereby reduce its viscosity. Typical materials that will bring about the foregoing objects are the alkali metal oxides, hydroxides, halides, carbonates, borates and compounds which decompose or burn to produce alkali metal oxides, and alloys consisting essentially of alkali metals. Examples of the foregoing include sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium fluoride, lithium fluoride, the chlorides of sodium, potassium and lithium, cryolite, sodium tetraborate, sodium oxide, potassium oxide, lithium oxide, the carbonates of potassium, lithium, cesium, rubidium and sodium, alkali metal organic compounds such as NaCZI-ls or NaOCH3 and similar compounds which are characterized in that the or their decomposition or combustion product, under the conditions specilied, are soluble in molten boric oxide. Such well-known intermetallic compounds and alloys as sodium-potassium is various known proportions, i.e. potassium content of about 1 to 50 weight percent, may be used for they result in the alkali metal oxides on combustion.

The compositions that are made in accordance with the present invention may be prepared in any manner desired. By way of example, where the boron fuel is liquid, the additive component is dissolved therein, if it is soluble, or simply mixed therewith if it is insoluble in the fuel. Simple mixing with sufficient stirring to provide a dispersal ofthe additive throughout the boron-containing material normally is satisfactory. Where the base fuel is a solid, the additive can be incorporated throughout the solid by use of any means which will effectively mix a solid material, such as a ball mill. Generally about 0.005 to y0.05 pound moles of the additive per pound atom of boron present is used. The use of greater amounts of additive does not appear to result in any advantage. in fact, excessive additive may be detrimental for it may increase the viscosity of the boric oxide and thus work against the desired result. i

Thus the fuels of the present invention comprise a boron-containing fuel and an additive as just described q in amounts of about 0.005 to 0.05 pound moles per pound atom of boron present. The invention is applicable to boron-containing fuels in general since they all result in boric oxide upon combustion. By boron-containing fuel we intend to indicate slurries of elemental boron in liquid hydrocarbons or a dispersal of boron in solid hydrocarbons, liquid and solid volatile or non-volative boron hydrides, and liquid or solid lower alkyl boranes such, for example, as triethylborane and tetraethyl diborane. T he materials contemplated thus are non-metallic, or metal free, and contain a minimum, on a weight basis, of about 5 percent boron. The invention is of particular significance with respect to boron base fuels composed essentially of pentaborane, decaborane, or lower alkyl substituted boranes such, for example, as ethyl substituted diboranes, tetraboranes, decaboranes, pentaboranes and mixtures thereof. Alkyl substituted boranes can be prepared by, for example, reacting the appropriate unsaturated hydrocarbon with a boron hydride in about a 1:1 ratio at an elevated temperature; an example involves reacting ethylene and tetraborane in a 1:11 mole ratio at a temperature of about 150 C.

Representative examples of fuel compositions in accordance with this invention are as follows:

mols of pentaborane-Q and 5 mols of sodium oxide; 95 Vmols of tetraborane and 5 mols of sodium tetraborate; 93 mols of decaborane and 7 mols of sodium hydroxide; 94 mols of ethyl decaborane and 6 mols of cryolite; 95 mols of a liquid mixture of ethyl substituted higher boranes such as ethyl decaborane, diethyl pentaborane and tetraethyltetraborane, plus 5 mols of potassium hydroxide; 88.3 weight percent of kerosene plus 10 weight percent of boron and 1.7 weight percent of sodium oxide.

Procedurally the fuel mixtures prepared in accordance with our discoveries may be used in the same manner as similar fuels, or the fuels without the additives, are now used. For example, the liquid fuels can be admixed with a stoichiometric quantity of an oxidizer, such as air or oxygen, and be fed to the combustion chamber of a jet engine. rEhe application of a spark or heat or other conventional combustion initiator can be used to burn the mixtures. The fuels also maybe metered to the combustion chamber of an internal combustion engine, along with a suitable oxidizer such as air or oxygen, and be combusted. The fuel may also be used in reaction motors such, for example, as that shown and described in the patent `to Malina et al., No. 2,573,471, by placing it in the lfuel container while using an oxidizer such as just mentioned in the oxidizer feed system and then feeding them products ofthe boron-containing fuel.

to the combustion chamber in the conventional manner. Solid fuels normally are used in rocket engines by dispersing a suitable oxidizer throughout the fuel and pack- Ying-the resulting mixture in the combustion area. Combustion of the mixture then is initiated, when the rocket is to lbe fired, by electrical or similarly conventional means. As is evident, the word engine as used in this specification is to be understood to mean jet engines, internal combustion engine, orrocket motors such as are now known. y, v

Another. advantageous method of practicing the invention involves incorporating the additive in the combustion In this embodiment the alkali metal additive, suitably as a liquid, is metered into the path of combustion products, for example in the exhausting passages closely adjacent the combustion zone, by a separate injection system. The additive solubilizes in the molten boric oxide, and facilitates its removal.

In order to discover quickly a chemical compound that would be satisfactory for the purposes herein described, a series of screening runs were made at `950 C. using l0 mole percent of various additives, in the form of the oxide or hydroxide, in pure boric oxide.

The apparatus used to test the effect of various compound-s on the viscosity of boric oxide is shown diagramv.matically in the attached drawing. A nickel plumb-bob 1%, long,'1/2 in diameter and tapered at both ends to 3/o,2 was suspended by means of a No. 30 gauge nichrome wire attached to a sensitive beam balance. AA 5 length of 1%. diameter Anickel pipe closed at one end was used as the crucibleto hold the boric oxide melt. The Crucible and its contents were heated in a Lindberg electric furnace equipped with a temperature indicator and controller. In using this apparatus, the boric oxide melt and the plumbbob were kept at a uniform predetermined temperature. The melt was well stirred to work out gas bubbles and to disperse the' additive. The plumb-bob While draining was suspended in saturated vapor above the melt to eliminate any error due to vaporization. 4 The bob was freely suspended from the balance to give rapidfand accurate response to weight changes.

In making a test run, sufficient pure glassy boric oxide v was added to the crucible to cover the bob when molten. The Crucible was placed in 'the furnace and as soon as the contents were molten, agitation of the crucible was started. After all the bubbles were worked out and the meltV thoroughly mixed and up to temperature, the nickel bob was lowered into the melt until the upper tip of the bob was just covered. Twice the time required to reach thermal equilibrium kbetween bob and melt was allowed to pass before raising the bob out of the melt and hanging it on the balance. The data obtained was the Weight of the bob plus adhering melt after `a delinite period of time measured from the time of initial withdrawal from the melt.l

The data obtained in a series of the screening runs are as follows:

lt is evident by an inspection of the above data that the most effective additives are the hydroxides of lithium, sodiumL and potassium as they decreased the adhesiveness of the melt by a factor of two compared to boric oxide itself.

, The hydroxides of the alkali metals previously screene were then studied in more detail. The table Vbelow shows the effect upon the viscosity of boric oxide obtained using l, 4 and l0 mole percent of these additives at temperatures of 650 C. and 950 C. after drain times of 1 and 5 minutes.

' Wt. Melt On Mol Per- Drain Bob (g.) Run Additive cent Time, Additive Mins.

0 1 8. 2O 2. 35 0 5 3. 82 1. 08 1 1 7. 40 1. 73 1 5 3. 02 0. 81 4 1 6. 60 1. 50 4 5 3. 15 0. 70 10 1 3.65 0. 96 l0 5 Y l. 97 0. 44 0 1 8. 20 2. 35 0 5 3. 82 1.08 1 1 5. 20 1. 65 1 5 2. 56 0. 8l 4 1 5. 10 1. 32 4 5 2. 50 0. 70 10 1 4. S2 0. 87 10 5Y 2. 35 0. 47 0 .1 8. 20 2. 35 O 5 3. 82 1. 08 1 1 5. 10k 1. 75 1 5 2. 57 0. 98 4 1 5. 50 l. 27 4 5 2. 77 0. G4 10 1 4. 30 1. 00 NaOH 10 5 2. 20 0.52

These data show that, for practical purposes, `the three additives are about equally effective in reducing the viscosity of molten boric oxide. Furthermore, there is little to be gained in using more than about five percent of the additive. However, in view of the fact that sodium compounds are the cheapest and most readily obtainable, such compounds would be the most logical choice as an additive of this type.

ln other experiments which were carried out, it was found that the carbonates, oxides, and halides (NaF and LiF in particular) of all the alkali metals as well as cryolite (Na3AlF6) and sodium tetraborate (Na2B4O7) were Viso. in Poises Mole Percent N aF From-these data it may be seen that at 750 C. viscosity lowering goes through a minimum point and that at high concentrations Vof sodium fluoride the viscosity is adversely affected. The larger quantities of the additive sharply increase the viscosity at 600 C. over that for the 6.4 percent concentration. V

When the alkali meal compounds described herein are incorporated in a high energy boron-containing fuel, such as pentaborane-9 or lower alkyl substituted boranes, and burned, they are effective in preventing `the deposition of boric oxide formed as the result of combustion since the fluidity of the resulting solution of boric oxide and the additive, or its decomposition or combustion product, is high and it flows away rather than accumulating. It

V should also benoted while the compound used as the additive need not be soluble in the fuel itself, since the fuel composition can be burned either as a slurry or as a solution of the additive compound in the fuel, the additive o1' its decompositions or combustion product must be soluble in molten B203. The solution of the additive in B203 so lowers the viscosity, or raises the iluidity, of the molten B203 that the B203 is readily blown from the walls of the equipment by the exhaust gases. Y

In accordance with the provisions of the patent statute we have explained the principle of our invention and have described what we now believe to represent its best embodiment. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

We claim:

1. In a method of operating an engine wherein a boron containing fuel and an oxidizer` therefor are introduced into a combustion zone of said engine and burned and the resulting exhaust from said combustion zone contains molten boric oxide which tends to build up along the exhausting zone of said engine, the improvement which comprises including in said fuel fed to said combustion zone an alkali metal component, in an amount of about 0.005 to 0.05 pound moles per pound atom of boron present which, after combustion of the resulting mixture, results in a solution of molten boric oxide of a viscosity lower than the viscosity of molten boric oxide alone.

2. In a method of operating an engine wherein a boron containing fuel and an oxidizer therefor are introduced into a combustion zone of said engine and burned and the resulting exhaust from said combustion zone contains molten boric oxide which tends to build up along the exhausting zonel of said engine, the improvement which comprises introducing a liquid alkali metal component into combustion products from said combustion zone, in an amount of about 0.005 to 0.05 pound moles per pound atom of boron present in the fuel entering said combustion zone, whereby a solution of molten boric oxide of increased fluidity is produced and is blown from said exhaust passages.

3. A method according to claim 1 in which said additive alkali metal component is a compound that decomposes to produce an alkali metal oxide.

4. A method according to claim 1 in which said additive alkali metal component is a compound which burns to produce an alkali metal oxide.

5. A method according to claim 1 in which said additive alkali metal component is an alkali metal oxide.

V 6. A method according to claim 1 in which said additive alkali metal component is an alkali metal hydroxide.

7. A method according to claim 1 in which said additive alkali metal component is an alkali metal halide.

8. A method according to claim 1 in which said additive alkali metal component is an alkali metal carbonate.

9. A method according to claim 1 in which said additive alkali metal component is an alkali metal borate.

References Cited in the iile of this patent UNITED STATES PATENTS 2,535,536 Hagmaier Dec. 26, 1950 2,573,471 Malina et al. Oct. 30, 1951 2,654,670 Davis et al. Oct. 6, 1953 2,744,380 McMillan et al. May 8, 1956 OTHER REFERENCES The Journal of Space Flight, vol. 2, No. 1 (Ian. 1950) pp. 1-9.

Hoekstra: The Preparation and Properties of Alkali Metal Borohydrides, AECD-2144, .Tune 1947, pp. 2, 3, 10, 16 and 18.

Claims (1)

1. IN A METHOD OF OPERATING AN ENGINE WHEREIN A BORON CONTAINING FUEL AND AN OXIDIZER THEREFORE ARE INTRODUCED INTO A COMBUSTION ZONE OF SAID ENGINE AND BURNED AND THE RESULTING EXHAUST FROM SAID COMBUSTION ZONE CONTAINS MOLTEN BORIC OXIDE WHICH TENDS TO BUILD UP ALONG THE EXHAUSTING ZONE OF SAID ENGINE, THE IMPROVEMENT WHICH COMPRISES INCLUDING IN SAID FUEL FED TO SAID COMBUSTION ZONE AN ALKALI METAL COMPONENT, IN AN AMOUNT OF ABOUT 0.005 TO 0.05 POUND MOLES PER POUND ATOM OF BORON

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