US2984556A - Fuel composition and process for its manufacture - Google Patents

Description

FUEL COMPOSITION AND PROCESS FOR ITS MANUFACTURE Samuel K. Talley, Berkeley, Calif., assignor to Shell Oil Company, a corporation of Delaware This invention is concerned with the improvement in the properties of ammonium nitrate and similar watersoluble crystalline oxidizing agents. More particularly, it is directed to a process for the preparation of an im proved ammonium nitrate and to its use in fuel compositions.

The utilization of ammonium nitrate as an oxidizing agent and for other purposes is limited by a number of its properties, such as the low viscosity of the material in its melted state. A second shortcoming of ammonium nitrate is its change in volume at relatively low temperatures (e.g., at about 32 C.), at which point the volume of the solid material increases about 4%. Another difliculty encountered with ammonium nitrate is its tendency to cake during storage, apparently due to absorption of moisture. Finally, particularly where it is utilized as an oxidant in fuel compositions, its rate of decomposition and combustion are lower than sometimes desirable.

Ammonium nitrate (and other oxidizing agents such as potassium perchlorate or ammonium perchlorate) is an available oxidizing agent which may be used in conjunction with combustible substances for such purposesas rocket fuels, jet assisted take-ofi fuels, and the like. In such systems the combustible material is preferably intimately mixed with the oxidizing agent and the mixture ignited to produce the propelling action necessary for rockets or other similar missiles. It has been determined that the rate of burning of such fuel compositions is much lower than desired for many potential applications and it has been further determined that this low rate of burning may be due to the adverse physical properties of both the combustible substance phase and the ammonium nitrate phase especially at or above their melt temperatures. Photographic studies have shown that rocket fuel compositions comprising asphalt and ammonium nitrate mixtures burn at a slow rate or actually cause snufiing of the flame at the burning phase, due to the'low viscosity of one or the other phase which coversthe burning face and thussnuflfs out the flame. A co-pending United States application by Samuel K. Talley, Serial No. 406,- 902, filed January 28, 1954, relates to the improvementin the properties of the combustible material (which is combined with ammonium nitrate and the like) whereby the viscosity and other properties of the combustible substance at or above its melting point is improved. The properties of the plastic combustible substance may be greatly altered by the presence of compatible or incompatible plasticizers therefor, or combinations of such types of plasticizers, and further by the incorporation of high temperature thickening agents. It has been found that combustible substances such as bituminousmaterials may be improved greatly with respect to their response to temperature change and more particularly in their viscosity properties at elevated temperatures.

While the properties of the plastic combustible materials thus can be improved markedly,- this leaves much to be desired with respect to the burning rates and other burningrqualitie's of suchcompositions whencombined ate-tit iee with ammonium nitrate or the like. While the properties of the phase containing the combustible material areimproved, it would appear to be necessary to improve the properties of ammonium nitrate at or above its'melt" temperature so as to obtain optimum burning properties.

It is an object of the present invention to improve the properties of ammonium nitrate and/or other oxidizing agents. It is a particular object of this invention to improve the properties of ammonium nitrate at or above its melt temperature. It is a further object of this invention to improve the burning properties of fuel compositions containing ammonium nitrate. It is another object of the invention to depress or alter the phase change Other objects willtemperature of ammonium nitrate. become apparent during their following discussion.

Now, in accordance with this invention, it has been the thickening eifect of high temperaturefound that thickening agents upon ammonium nitrate is substantially increased by combining the thickening agent with an aqueous solution of the nitrate prior to dehydrating the latter (rather than by dry mixing). Still in accordance with this-invention, further improvements in the properties of ammonium nitrate may be made by the further modification of the substance with potassium nitrate and/ or ammonium dichrornate.v Again in accordance with this'invention, it has been found that ammonium nitrate compositions modified by the process outlined above provide especially desirable properties for their incorpo ration in fuel compositions and especially in rocket fuel compositions.

The properties of ammonium nitrate have been modified by the dry mixing of nitrate with various other sub-" stances such-as Prussian blue and the like. However it' has been ascertained that the improvement in ammonium nitrate caused by such method of incorporation is only of a minor order and that the improvement is not suificient to besatisfactory. The present process comprises addition'ofPrussian blue and other high temperature thickening agents outlined hereinafter to ammonium' nitrate (or inorganic perchlorates) while the latter is inaqueous solutionandthereafter dehydrating the mixture thus formed. The-incorporation of the modifying substances by this method produces ammonium nitrate having:

properties, especially -at its melt or burning temperatures,- substantially better than those obtained by the dry mixing;

of the separate ingredients.

In carrying out the process of the present invention, it

is necessary to have only sufficient water present to 0 a liquid phase into which the modifying ingredients are mixed or in which-the ingredients are formed. At elevated temperatures in the order of 200 F. and above, I

very small percentages of water in the order of 25% of water cause the formation of liquid solutions with am monium nitrate. Consequently, the ammonium nitrate may be dry at room temperature, or apparently so, but

may contain suflicient water to be in a liquid state at anelevated temperature and suitable for use in the present process. However, the process is not confined to the use of systems in which such small proportions of water exist. On the contrary, it is possible to use any greater amount 'of water desired, for example, 30-70% of water based w on the total of water and ammonium nitrate. Under 'theseconditions, of course, the temperature atwhichliquid solutions exist is of a considerably lower order, such as between room temperature and 200 F. The dehydrationmay be carried out by any desired means. If minimum particle size of the dried material is desired, the-solution of the water-soluble ingredients may be emulsified with awater-immiscible (preferably volatile) liquid, using 15% based on the solids, of anemulsifying agent capable of forming a water-in-oil type Patented lviay 16, 1961 of emulsion; dehydrating the emulsion and removing the water-immiscible liquid.

The modifying ingredients which may be employed in the present process for the purpose of increasing the viscosity of the molten ammonium nitrate include especially amorphous inorganic colloidal substances, clay materials as defined hereinafter, carbon blacks and Prussian blue.

The clay materials useful in the present process and for the preparation of the novel compositions more fully described hereinafter include especially high base exchange clays, onium-clays (reaction products of natural clays with onium compounds) and natural clays bearing hydrophobic substances absorbed on their surfaces. The naturally occuring materials are mainly those of clay-like nature and preferably comprise the swelling type of bentonite clays. While the swelling type of clays are preferred, the non-swelling types such as Georgia kaolinite also may be used. Optimum compositions may be obtained by the use of magnesium montmorillonite, such as hectorite. The Wyoming bentonite ordinarily regarded as aluminum montmorillonite, also is highly effective.

Synthetic materials may be prepared having properties suitable for use in the present process and similar to the naturally occurring materials insofar as their present function is concerned. Since they are so closely similar to the naturally occurring clays, they are for the present specification and claims to be regarded as included in the term clay materials. These synthetic materials are also called synthetic zeolites. The purposes for preparing and using synthetic clay-like materials are to avoid the presence of non-colloidal impurities if this is desirable, or at least to have under close control the exact composition of the colloid, e.g., the relative proportions of silica, alumina and sodium oxide or other oxide which may be varied Within wide ratios. The preferred variety of synthetic zeolite comprises alumina-silicate which may be regarded as compounds of alumina and silica containing varying ratios of sodium oxide.

Onium clays are normally prepared by reaction of an onium compound such as a quaternary ammonium salt with a clay having a relatively high base exchange capacity, generally between about 25 and about 100 milliequivalents of exchangeable base per hundred grams of clay. The onium compounds such as those described hereinafter are of such a type and are employed in such a proportion as to provide an onium clay reaction product swelling at least eight-fold in nitrobenzene. Compositions of this character are described in the Hauser patent, US. 2,531,427.

Onium compounds which may be utilized in the formation of onium clays include, for example, naphthylammonium chloride, dicyclohexylammonium bromide, lauryldimethylammonium chloride, tetraethanolammonium bromide, octadecylammonium iodide, and laurylphosphonium bromide as well as dimethyldicetylammonium bromide.

In addition to the naturally occurring (or synthetic) clay-like materials and their onium derivatives, another variety of high temperature thickener may be prepared by simple adsorption of a hydrophobic agent on the surfaces of the clay without any substantial degree of chemical reaction therewith. In the preparation of hydrophobic clay materials of this class, primary, secondary or tertiary alkyl amines and other aliphatic amines are preferred. These include such amines as dodecyl amine, hexadecyl amine, octadecyl amine and mixtures thereof. Preferably, the amines should contain at least ten carbon atoms and still more preferably at least ten carbon atoms in a single aliphatic hydrocarbon chain. Other suitable means useful in these compositions include cyclic amines such as naphthylamine, heterocyclic amines such as pyridine and pyrimidine derivatives and tetrahydro pyrimidines, as well as, polyamines and amidoamines.

' The carbon blacks which may be employed in improving the properties of ammonium nitrate include such materials as furnace blacks and channel blacks, the former being characterized by generally smaller surface areas and larger particle sizes than the latter. Furnace black surface areas range from about 3 to about 100 square meters per gram, equivalent to about 0.4 to 0.5 micron average particle diameter. On the other hand, channel blacks range from about 100 to about 200 square meters per gram, equivalent to 0.2-0.4 micron average particle diameter. These blacks are known in the trade under other names, such as channel black, acetylene black lamp black and thermal black. Mixtures of the blacks may be ob tained in order to modify the thickening properties of molten ammonium nitrate.

Amorphous inorganic colloidal materials suitable for the present purpose include silica, alumina, magnesia;

vanadium oxide, and ferric oxide, as well as the corre snonding hydroxides (including lime), alkaline earth carbonates, such as calcium carbonate, etc. Preferably these inorganic colloidal substances are in the form of highly porous materials typified by (but not exclusively restricted to) aerogels and the like. Preferably the amorphous colloids have surface areas in the order of 100 square meters per gram or higher. They may be prepared by a number of well-known methods, such as the burning of silicon tetrahalide to form finely divided silicon oxide, or may be formed by solvent replacement, such as the replacement of water from an inorganic hydrogel with alcohol and subsequent removal of the alcohol by heating the alcogel in an autoclave to a temperature above the critical temperature of the alcohol and thereafter releasing the pressure so as to form a highly porous gel sometimes referred to as an aerogel. Another material suitable for the present purpose is Prussian blue.

Modification of ammonium nitrate with such materials as 5% by Weight of jewelers rouge or calcium silicate (or other modifiers such as those described above) by dry mixing of the ingredients resulted in no apparent advantage, since the mixture, upon melting was very fluid. However, addition of the same materials to a aqueous solution of ammonium nitrate, followed by dehydration, resulted in mixtures which were pasty in their melted state. Furthermore, the latter compositions did not readily separate from asphalt at the melt temperatures, when such compositions were being burned.

One of the disadvantages of ammonium nitrate comprises its volume change at various temperatures. An important phase change temperature is about 32 C., a temperature commonly encountered in the storage and use of shaped charges such as rocket fuels. This volumetric change is disadvantageous due to the fact that it causes cracking of the charge and thereby results in uneven burning thereof. Certain materials may be added to ammonium nitrate to minimize or alter this phase change. Potassium nitrate may be added for the purpose of increasing the temperature at which the volumetric change occurs to a limited extent. It has been found, in accordance with one phase of this invention, that the addition of the above defined thickening agents also has a retarding effect upon the phase change and may show a synergistic effect when combined with potassium nitrate. Table I below illustrates these properties.

Table 1 Volume Change Additive Temperature,

The addition of Prussian blue increased the volume change temperature by 10 degrees. Combination of Prussian blue with potassium nitrate caused a 16 degree rise in the'volume change temperature while silica and alumina causedphase change temperatures'ofand 6- C., re-" described hereinbefore, causes catalysis of the oxidizing.

action of the nitrate. Not only is oxidation'catalysed, but the stiffening agents appear to act as synergists when combined with other catalysts, such as ammonium dichromate. This will become apparent in the burning tests to be described hereinafter.

Aspointed out hereinbefore, one of the principal uses of modified ammonium nitrate. prepared as described above is in the compounding of fuels and particularly of guided missile or rocket fuels. Broadly, the class of fuelsconsidered here are plastic solids comprising a combustible material intimately commingled with'an'oxidizing agent and providing the propellant power for rockets and the like to be used in missiles or for jet aircraft takeoff or for added power during aircraft flight. Theprincipal combustible materials considered for use in these plastic solid fuels include bituminous substances, organic polymers, including resins or rubbers, and cellulose compounds, including cellulose ethers or esters, such as ethyl cellulose, cellulose acetate and cellulose nitrate. These solid combustible materials perform the dual function of fuels and plastic materials capable of being molded together with the oxidizing agent into a plastic solid fuel charge. Therefore, an important aspect of the present invention comprises the utilization of the modified ammonium nitrates (or other oxidizing agents, such as modified ammonium perchlorate) in plastic solid fuels of this type.

The bituminous substances which may be employed in the subject plastic solid fuels comprise for the most part those of petroleum origin or derived from coal tar or coal tar pitch. The essential requirement for the compositions under consideration is that the bituminous substance have an ASTM penetration at 77 F. (five seconds, 100 gram load) less than about 20 dmm. and preferably less than 10 dmm. This is then plasticized to yield a binder fuel having an ASTM penetration of about 150-250 dmm. at 77 F. The bituminous materials also should have an ASTM ring and ball softening point between about 150 and about 300 F., and preferably between 175 and 225 F. Suitable materials may be prepared by the use of one or more asphaltic or coal' tar components and may be either blown or unblown petroleum products.

An asphalt suitable for the present purpose may be prepared, for, example, by'air blowing of'either distilled or residual petroleum fractions. For example, short residue, heavy bulk distillates; lubricating-oil extracts or bright stocksmay be blown to'suitable' asphaltic compositions having a desired penetration range. Preferably, the starting materials contain only a minor amount of aromatic fractions, and short residues obtainedfrom the West Texas fields are preferred. It is still more preferred that asphalts are employed having softening points above about 285 F. and are the dewaxed residue of a mixed base crude. In addition to the asphalts and'coal tar pitches suitable for use in these compositions, asphaltites,

such as gilsonite and grahamite may be'employedeither' as the sole bituminous substance or in combination with one or more of the above materials.

As discussed in the'co-pending application of Samuel K. Talley, Serial No. 406,902, of which this case is in part a continuation-in-part, one of the-serious shortcomings ofa number of these plastic fuels comprises their highly fluid character when in themolten state; Accordingly, it'was found that'certain stilfeningpgents could be employed to overcome or minimize this de ficiency and comprise the general class of stiffening agents described hereinbefore' and which are-now being: added" to the ammonium nitrate phase.

It has been'found that the addition of the stiffening agent 'to one-phase only is not sutficient to cause stiffening of both phases, but that the stiffening agents are most effective if separately incorporated in each phase prior recombination of the two phases in the final assembly of the fuel composition.

Furthermore, certain of the fuels, such as the bituminous materials described above, exhibit disadvantageous properties with respect to change in viscosity and consistency with change in temperature. It has been found that these properties can be radically altered by-rnodification of the bituminous material with certain esters or mineral lubricating 'oils.

The esters are fully described in the co-pending parent application and generally comprise phosphorus esters (particularly, normally liquid aliphatic esters of phosphorus; organic silicate esters and aliphatic esters of polycarboxylic acids). Typical esters include the trialkyl phosphates, such as trioctyl phosphate, trihexyl phosphate,-tridodecyl phosphate and other phosphates contain-- ing a total of at least eighteen carbon atoms per molecule and preferably between about 20 and 32 carbon atoms per molecule. The corresponding phosphinates and phosphonates may be employed, such as di(2-ethylhexyl)hexane phosphonate. Silicate esters include alkyl,

aryl, aralkyl and alkylaryl esters of silicic acid. Typical members of these classes include tetra(2-ethylhexyl)silicate and-tetracresyl silicate. Suitable dicarboxylic esters include di(2-ethylhexyl)sebacate and other typical aliphatic esters, such as those disclosed in U.S. Patent 2,481,372

As the parent'application of Samuel K. Talley, referred to above, shows, a combination of high temperature thickening agents and the plasticizing materials produces a particularly. beneficial type of bituminous composition. This is especially suitable for its low response to temperature changes such as may be required in the use of the bituminous compositions as fuels in combination with the modified nitrates (or perchlorates). Consequently, an important feature of this invention comprises the combinationof. the modified ammonium nitrates prepared by the wet process described hereinbefore combined with bituminouscompositions.containing both a high temperature thickening'agent and a plasticizing'material of the classdescribed. Preferred compositions containing these two'modified phases are as follows:

The practical significance of the modification of both the ammonium nitrate phase and the plastic fuel phase substantially as described above is readily determined in the relative burning'rates of compositions omitting or containing the modifying substances as demonstrated by the'data given hereinafter. Burning strands having a quarter inch cross-section were compounded by intimately mixing the'bitumin-ous fuel phase and the ammonium nitrate oxidant phase and burning them in a nitrogen or inert atmosphere at one thousand pounds per square inch gauge pressure. If such strands are prepared by mixing fifteen parts by weight of a hard blown asphalt withpartsbyweight of finely ground ammonium nitrate, the

spawns resulting flame is quickly snuffed out due to the melting and separation of the components which retard or even prevent the ammonium nitrate from contacting the burning phase. Modification of the asphalt with 1% by weight 3'. A fuel composition according to claim 1 wherein potassium nitrate also is dispersed throughout the ammonium nitrate phase.

4. A' fuel composition according to claim 1 wherein of the total composition of the reaction product of bento- 5 the asphalt phase comprises an oxidized asphalt material. nite and dicetyldimethylammonium chloride (Bentone 5. A fuel composition comprising predominantly 34 trade name) resulted in a burning rate of 0.06 inch 40% by weight of an asphalt phase having colloidally disper second. The use of 5%, based on the total composipersed therethrough a clay material and a separate phase tion, of a catalyst (ammonium dicromate) mixed with the commingled therewith comprising 60-90% by Weight of ammonium nitrate phase and of 1% (based on the total 10 ammonium nitrate having dispersed therethrough a thickcomposition) of the same onium clay dispersed in the ening agent selected from the group consisting of colloidal asphalt phase, resulted in a burning rate substantially silica gel, clay, ammonium clays capable of swelling at twice that of the second sample, namely, 0.127 inch per least 8-fold in nitrobenzene, clays bearing hydrophobic second. However, by incorporation of a thickening agent amines adsorbed on the surfaces thereof, carbon blacks in the ammonium nitrate phase by the wet method deand Prussian blue; the proportion of thickening agents scribed hereinbefore, the burning rate was increased to being sufiicient to substantially thicken both phases when nearly triple that of the second sample described above. they are melted, said agent constituting 1-10% by weight Wet mixing of potassium nitrate with ammonium nitrate of the asphalt phase and 2-10% by weight of the arnmodid not result in any burning rate advantage. The follownium nitrate phase. ing table gives typical results of such experiments: 6. In a method for the production of improved fuel Wet Mix Amount, Avg. Dry Mix Sample Percent Component Burning Burning w. Rate, Rate in./sec.

14 AsphaltC A 1 Bentone 34... 127

"""" 8O Ammonium mtrate, 3X ground 5 Ammonium dichromate, ground 14 AsphaltC 1 Bentone 34 B 85. 4 ammonium nitrate 151 .116

85 4.3 Prussian blue }3X ground 10. 3 potassium nitrate 14 AsphaltO 1 Bentone 34 O {85. 4 ammonium nitrate} 208 80 4.3 Prussian blue 3X ground.

10.3 potassium nitrate 5 Ammonium dichromate, ground 14 AsphaltO 1 Bentone 34 D 91.0 ammonium nitrate .172 85 4.5 furnace black }3X ground 4. 5 potassium nitrate 14 AsphaltG 1 Bentone 34 E {91.0 ammonium nitrate 188 85 4. 5 furnace black 3X ground 4. 5 potassium nitrate 5 Ammonium dichromate, gr0uJ1d 14 Asphalt C"..- F 1 Bentone 34 101 i B t 34 111 118 G i 85 {95 a1iim0nium nitrate" 10 potassium nitrate Asphalt C, used in the above rocket fuels, comprised compositions, the steps comprising combining ammonium 64% by weight of a commercial blown asphalt (penetranitrate in aqueous solution with a thickening agent and tion=19 dmm. at 77 F.) combined with 36% by weight thereafter dehydrating the combined components whereby of a West Texas mineral lubricating oil having a viscosity a thickened ammonium nitrate phase is formed; combining of about 100 SUS at 100 F. and a viscosity index of an asphalt with a thickening agent whereby a thickened about 90. asphalt phase is formed; and milling -90 percent by I claim as my invention: 60 weight, based on the total composition, of the thickened 1. A fuel composition consisting essentially of an ammonium nitrate phase with 40-10 percent by weight, asphalt phase and a separate ammonium nitrate phase based on the total composition, of the thickened asphalt commingled therewith, each phase having intimately disphase whereby a fuel composition is formed; the thickenpersed therethrough a thickening agent of the group coning agent present in each of the phases being selected sisting of colloidal silica gel, clay, ammonium clays capav from the group Consisting 0f colloidal Silica g y, ble of swelling at least 8-fold in nitrobenzene, clays bearammonium clays capable of swilling at least 8-fold in ing hydrophobic amines adsorbed on the surfaces thereof, nitrobenzene, clays having a hydrophobic amine adsorbed carbon blacks and Prussian blue, the amount, of thickenon the surfaces thereof, carbon blacks and Prussian blue, ing agent present in each phase being at least sufiicient to the proportion of thickener being 2-10% by weight in the substantially thicken the phases when they are melted, ammonium nitrate phase and 1-10 percent by weight in said agent constituting 1-10% by weight of the asphalt the asphalt phase. phase and 2-10% by weight of the ammonium nitrate 7. A fuel composition comprising an asphalt phase phase. and a separate ammonium nitrate phase commingled 2. A fuel composition according to claim 1, wherein therewith, the asphalt phase having dispersed therethrough ammonium dichromate also is dispersed throughout the 2-10% by weight of an ammonium clay capable of swellnitrate phase.

ing at least 8-fold in nitrobenzene, and the ammonium 9 nitrate phase having 1-10% by weight of Prussian blue 2,434,872 dispersed therethrough. 2,531,427 2,661,301 References Cried 1n the file of th1s patent UNITED STATES PATENTS 5 1,021,882 OBrien Apr. 2, 1912 14,196 2,159,234 Taylor May 23, 1939 665,585

10 Taylor et a1 Jan. 20, 1948 Hauser Nov. 28, 1950 Capell Dec. 1, 1953 FOREIGN PATENTS Great Britain 1897 Great Britain July 25, 1951

Claims (1)

1. A FUEL COMPOSITION CONSISTING ESSENTIALLY OF AN ASPHALT PHASE AND A SEPARATE AMMONIUM NITRATE PHASE COMMINGLED THEREWITH, EACH PHASE HAVING INTIMATELY DISPERSED THERETHROUGH A THICKENING AGENT OF THE GROUP CONSISTING OF COLLOIDAL SILICA GEL, CLAY, AMMONIUM CLAYS CAPABLE OF SWELLING AT LEAST 8-FOLD IN NITROBENZENE, CLAYS BEARING HYDROPHOBIC AMINES ADSORBED ON THE SURFACES THEREOF, CARBON BLACKS AND PRUSSIAN BLUE, THE AMOUNT OF THICKENING AGENT PRESENT IN EACH PHASE BEING AT LEAST SUFFICIENT TO SUBSTANTIALLY THICKEN THE PHASES WHEN THEY ARE MELTED, SAID AGENT CONSTITUTING 1-10% BY WEIGHT OF THE ASPHALT PHASE AND 2-10% BY WEIGHT OF THE AMMONIUM NITRATE PHASE.