WO2008082677A2

WO2008082677A2 - Hybrid rocket fuel

Info

Publication number: WO2008082677A2
Application number: PCT/US2007/062292
Authority: WO
Inventors: Robert Freerks
Original assignee: Syntroleum Corporation
Priority date: 2006-02-17
Filing date: 2007-02-16
Publication date: 2008-07-10
Also published as: WO2008082677A3

Abstract

A fuel grain is disclosed that comprises a core having a longitudinal axis and at least one internal aperture substantially aligned with the longitudinal axis wherein the core comprises a fuel composition comprising first and second fuel components. In some embodiments, the ratio of the first fuel component to the second fuel component at a first position relative to the longitudinal or latitudinal axis is greater than the ratio of the first fuel component to the second fuel component at a second position relative to the longitudinal axis or latitudinal. In another embodiment, the fuel grain comprises a fuel composition that includes less than 1 ppm sulfur. Some compositions use a Fischer-Tropsch wax as one or more of the fuel composition components. Methods of making such fuel grains are also described.

Description

HYBRID ROCKET FUEL

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S Provisional Application No. 60/774,702, filed on February 17, 2006.

FEDERALLY SPONSORED RESEARCH

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

FIELD OF THE INVENTION

The invention relates to hybrid rocket fuel systems and apparatuses for using hybrid rocket fuels.

BACKGROUND OF THE INVENTION

Two basic types of chemical rocket propulsion systems are widely used in the rocket industry. One type is the liquid propulsion system wherein a liquid oxidizer and liquid fuel are fed to a combustion chamber where they mix and react. The resulting high temperature, high pressure gases are expelled through a converging-diverging nozzle producing thrust. Generally, rockets using liquid propellant systems require complex systems for propellant delivery and exotic combustion chamber designs, making such systems capital intensive.

The other basic type of propulsion system is the solid propulsion system. In the solid propulsion system, solid fuel and oxidizer are intimately mixed and allowed to cure inside the rocket case. Unlike a liquid system, where the feed of one of the liquid components can be controlled, once the solid fuel has been ignited, it burns uninterrupted until all the fuel is exhausted. And while solid propellant systems do not require the capitally intensive machinery of liquid systems, solid propulsion systems are subject to the difficulties of consistently producing crack-free fuel grains. Fuel grains which contain cracks present a risk of uncontrolled burning upon ignition or at another point during the burning process. In addition, solid propulsion systems also present the risk of transporting and handling inherently explosive materials.

An alternative propulsion system is known as the hybrid propulsion system. In the hybrid design, one propellant is stored in the solid phase while the other is stored in the liquid

HOUDMS/204889.1 phase. In most hybrid propulsion applications, the solid is the fuel and the liquid is the oxidizer. Reverse hybrids with the fuel in the liquid phase and oxidizer in the solid phase are also feasible. However, there is still a need to provide hybrid propulsion systems that have improved thrust and burn properties.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to a fuel grain that includes a core having a longitudinal axis and at least one internal aperture substantially aligned with the longitudinal axis. The core comprises a fuel composition comprising first and second fuel components. The fuel grain of some embodiments comprises a compositional gradient of the first or second fuel component with respect to the longitudinal axis of the fuel grain. In some embodiments, the ratio of the first fuel component to the second fuel component at a first position relative to the longitudinal axis is greater than the ratio of the first fuel component to the second fuel component at a second position relative to the longitudinal axis. Some embodiments provide a fuel grain wherein the first fuel component has a lower viscosity or higher molecular weight than the second fuel component.

In some embodiments of the invention, the core includes an upper section and a lower section. The fuel composition in the upper section differs from the fuel composition in the lower section. Some embodiments provide a fuel grain wherein the fuel component of the upper section has a lower viscosity or lower molecular weight than the fuel component in the lower section.

In other embodiments, the relative amount of the first fuel component at a first position relative to the latitudinal axis of the fuel grain core differs from the relative amount of the first fuel component at a second position relative to the latitudinal axis of the fuel grain core. The relative amount of the first fuel component is based on the combined weights of the first and second fuel components. In some embodiments, the fuel grain comprises a compositional gradient of the first or second fuel component with respect to the latitudinal axis of the fuel grain.

In some embodiments, the core includes an inner section and an outer section. The fuel composition in the inner section is different than the fuel composition in the outer section. Some embodiments provide a fuel grain wherein the fuel component of the inner section has a lower viscosity or lower molecular weight than the fuel component in the outer section. Some

HOUDMS/204889.1 embodiments provide a fuel grain wherein the first fuel component has a lower viscosity or higher molecular weight than the second fuel component.

Other embodiments of the invention provide a fuel grain that includes a core comprising a fuel having less than about 1 ppm of sulfur, wherein the core includes a longitudinal axis and at least one internal aperture substantially aligned with the longitudinal axis. In some embodiments the fuel having less than 1 ppm of sulfur is selected from waxes produced by a Fischer-Tropsch process.

Embodiments of the invention also disclose methods of making such fuel grains. In some embodiments the method includes providing a first fuel component, providing a second fuel component, and forming a core comprising the first and second fuel components, wherein the core has a longitudinal axis and at least one internal aperture substantially aligned with the longitudinal axis. Some embodiments of the methods described herein include forming a core in a manner that provide a first ratio of the first fuel component to the second fuel component at a first position relative to the longitudinal axis and providing a second ratio of the first fuel component to the second fuel component at a second position relative to the longitudinal axis.

In other embodiments, the methods include forming a core having at least an upper section and a lower section wherein the lower section of the core is substantially free of the first fuel component. In some embodiments, the relative amount of the first fuel component at a first position relative to the longitudinal axis differs from the relative amount of the first component at a second position relative to the longitudinal axis, wherein the relative amount of the first fuel component is based on the combined weights of the first and second fuel components.

In some embodiments of the methods described herein forming the core includes forming the core to provide a relative amount of the first fuel component at a first position relative to the latitudinal axis of the core that differs from the relative amount of the first fuel component at a second position relative to the latitudinal axis of the core. The relative amount of the first fuel component is based on the combined weights of the first and second fuel components.

Some methods described include forming a core comprising a fuel composition having a sulfur content of less than about 1 ppm, wherein the core has a longitudinal axis and at least one internal aperture substantially aligned with the longitudinal axis. In some embodiments, the fuel is a wax prepared by a Fischer-Tropsch processes.

HOUDMS/204889.1 In some of the embodiments described herein, the core comprises at least an inner section and an outer section; and wherein the inner section comprises from 0 to about 100 percent by weight of the first fuel component and the outer section comprises from 0 to about 100 percent by weight of the second fuel component.

Some embodiments may include three or more fuel components in the fuel composition. Any number of fuel components may be included and provided in various amounts in the fuel composition. Like the first and second fuel components, the amount of any additional fuel components may, but need not, also vary according to the position with respect to the longitudinal or latitudinal axis of the core of the fuel grain, whether or not the fuel grain comprises distinct sections.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram of a fuel grain according to one embodiment of the invention.

Figure 2 is a schematic diagram of a fuel grain having both a longitudinal axis and a latitudinal axis according to one embodiment of the invention.

Figure 3 is a schematic diagram of a fuel grain having an inner section and an outer section according to one embodiment of the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following description, all numbers disclosed herein are approximate values, regardless whether the word "about" or "approximate" is used in connection therewith. They may vary by 1 percent, 2 percent, 5 percent, and sometimes, 10 to 20 percent. Whenever a numerical range with a lower limit, R^L and an upper limit, R^u, is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=R^L+k*(R^u-R^L), wherein k is a variable ranging from 1% to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent,..., 50 percent, 51 percent, 52 percent,..., 95 percent, 96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed.

The term "C_x"_, where x is a number greater than zero, refers to a hydrocarbon compound having predominantly a carbon number of x. As used herein, the term C_x may be modified by reference to a particular species of hydrocarbons, such as, for example, C₅ olefins. In such

HOUDMS/204889.1 instance, the term means an olefin stream comprised predominantly of pentenes but which may have impurity amounts, i.e. less than about 10 percent, of olefins having other carbon numbers such as hexene, heptene, propene, or butene. Similarly, the term "C_x+" refers to a stream wherein the hydrocarbons are predominantly those having a hydrocarbon number of x or greater but which may also contain impurity levels of hydrocarbons having a carbon number of less than x. For example, the term C₁₅₊ means hydrocarbons having a carbon number of 15 or greater but which may contain impurity levels of hydrocarbons having carbon numbers of less than 15. The term "C_x-C_y"_; where x and y are numbers greater than zero, refers to a mixture of hydrocarbon compounds wherein the predominant component hydrocarbons, collectively about 90 percent or greater by weight, have carbon numbers between x and y inclusive. For example, the term C₅- Cg hydrocarbons means a mixture of hydrocarbon compounds which is predominantly comprised of hydrocarbons having carbon numbers between 5 and 9 inclusive, but may also include impurity level quantities of hydrocarbons having other carbon numbers.

The phrase "substantially free" as used herein means that the component to which this phrase refers comprises less than about 5 percent of the composition. In some embodiments, the phrase "substantially free" means that the component to which the phrase refers is present in only trace amounts or the level of the component was not intentionally increased over the amounts present in the commercial source of other components of the total composition.

As used herein the term "compositional gradient" refers to a change in any property of the fuel composition with respect to its position or location in the fuel grain or with respect to a characteristic of the fuel grain, such as a longitudinal axis or latitudinal axis. In some embodiments "compositional gradient" means that a relative amount of one or more fuel components changes as a function of position within the fuel grain. In other words, in some embodiments the term "compositional gradient" means that the difference between the amount of at least one fuel component at one arbitrary position in the fuel grain is at least about 5, about 10, about 20, about 25, about 30, about 40, about 50, or about 75 percent by weight different than the amount of that component at a second arbitrary position in the fuel grain. In some embodiments the term "compositional gradient" means that the viscosity of the fuel composition changes as a function of position within the fuel grain. Thus, in some embodiments, the term "compositional gradient" means that the difference between the viscosity of the fuel composition comprising the first and second fuel components at one arbitrary position in the fuel grain is at least about 10,

HOUDMS/2Q4889.1 about 25, about 50, about 75, about 100, or about 200 percent greater than the viscosity of the fuel composition at a second arbitrary position in the fuel grain. The term compositional gradient may also mean that the average molecular weight of the fuel components in the fuel grain changes with respect to location in the fuel grain. The average molecular weight of the components is usually determined using the individual molecular weights of the paraffinic hydrocarbons in the fuel composition. Thus, in some embodiments, the average molecular weight of the fuel composition at one arbitrary point in the fuel grain is at least about 10, about 20, about 25, about 30, about 40, about 50, about 75, about 100, or about 200 percent different the amount of that component at a second arbitrary position in the fuel grain. In some embodiments, the term "compositional gradient" means that there is an a incremental change in the amount of either the first or the second fuel component. The term "compositional gradient," however, need not mean a smooth or constant change in the amount of a fuel component, viscosity of the fuel composition, or the average molecular weight of the fuel composition is present or determined at more than two positions in the fuel grain.

Unless otherwise specified, all quantities, percentages and ratios herein are by weight.

Embodiments of the invention are directed to a fuel grain 1 , an exemplary illustration of which is depicted in Figure 1 , that includes a core 2 having a longitudinal axis 3 and at least one internal aperture 4 substantially aligned with the longitudinal axis. The core is made up of a fuel composition that has at least two different components. In some embodiments, at least one of the fuel components comprises a wax produced by a Fischer-Tropsch process. Typically, the different components form a compositional gradient with respect to the longitudinal axis of the fuel grain. It can be desirable for one fuel component to have a lower viscosity or higher molecular weight than the other fuel component. In some embodiments, the fuel grain 1 can be considered to have an upper section 5 and a lower section 6, although the upper section 5 and lower section 6 need not comprise equal portions of the fuel grain 1.

With continuing reference to Figure 1 , some embodiments of the invention described herein include a fuel grain 1 comprising a fuel composition that includes a first fuel component having a lower viscosity than a second fuel component. In some embodiments, the difference between the viscosities of the first and second fuel component ranges has an upper limit of about 25, about 40, about 50, about 75, about 100, or about 200 percent. The lower limit on the range of difference in viscosities in some embodiments, is about 10, about 25, about 40, about 50,

HOUDMS/204889.1 about 75, or about 100 percent. In some embodiments the viscosity of the fuel composition at a first point 7 is lower than the viscosity of the fuel composition at a second point 8 in the fuel cell. In other embodiments the viscosity at the first point 7 may be higher than at the second point 8, depending on the burn characteristics desired for the fuel grain. Fuel compositions having viscosity differences outside these ranges are envisioned. Thus, any fuel grain wherein the viscosity of the fuel composition at a first point in the fuel grain is different that the viscosity of the fuel composition at a second position in the fuel grain is envisioned. Typically, the viscosity of the components should be selected to provide desirable oxidation rates during the planned burn cycle with the understanding that the upper portions of fuel grain 1 are generally consumed before the lower portions.

In some embodiments, the fuel grain includes a fuel composition having at least two fuel components, wherein the first fuel component has a lower molecular weight than the second fuel component. In some embodiments, at the first point 7 the first fuel component comprises paraffinic hydrocarbons having from about 15 to about 100 carbon atoms. In particular embodiments, the first fuel component comprises paraffinic hydrocarbons having from about 18 to about 75 carbon atoms. In other embodiments, the lower limit on the range of carbon atoms of the paraffinic hydrocarbons in the first fuel component is about 20, about 25, about 30, about 35, about 40, or about 50 carbon atoms. In some embodiments, the upper limit on the range of carbon atoms of the paraffinic hydrocarbons in the first fuel component is about 100, about 95, about 90, about 80, about 70, about 60, about 50, or about 40 carbon atoms. In particular embodiments, the second fuel component comprises paraffinic hydrocarbons having from about 20 to about 120 carbon atoms. In particular embodiments, the second fuel component comprises paraffinic hydrocarbons having from about 25 to about 100 carbon atoms. In other embodiments, the lower limit on the range of carbon atoms of the paraffinic hydrocarbons in the second fuel component is about 35, about 40, about 50, about 60, about 70, about 80, about 90 or about 95 carbon atoms. In some embodiments, the upper limit on the range of carbon atoms of the paraffinic hydrocarbons in the second fuel component is about 115, about 110, about 100 about 95, about 90, about 80, about 70, about 60, about 50, or about 40 carbon atoms.

In some embodiments, at least the first or second fuel component of the fuel composition is a low sulfur composition. Low sulfur compositions typically are waxes having less than about 1 ppm of sulfur. In some embodiments, the low sulfur waxes have less than 0.5 ppm, less than

HOUDMS/204889.1 0.25 ppm, less than 0.10 ppm, less than 0.05 ppm, or less than 0.001 ppm of sulfur. Typically, lower sulfur concentrations are preferred for environmental reasons, so preferably both the first and second are low sulfur compositions.

In some embodiments, the fuel grain 1 further comprises a third fuel component, wherein the relative amount of the third fuel component at the arbitrary first position 7 relative to the longitudinal axis differs from the relative amount of the third fuel component at the second position 8 relative to the longitudinal axis. The relative amount of the third fuel component is based on the combined weights of the second and third fuel components. In some embodiments wherein the fuel grain includes the third fuel component, the relative amount of the first fuel at the first position 7 relative to the longitudinal axis 3 differs from the relative amount the first fuel component at a second position 8 relative to the longitudinal axis 3; and the relative amount of the first fuel component is based on the combined weights of the first and third fuel components.

In some embodiments, the fuel grain 1 comprises at least the upper section 5 and the lower section 6 wherein the first fuel component comprises from 0 to about 100 percent by weight of the fuel composition in the upper section 5; and wherein the lower section 6 comprises from 0 to about 100 percent by weight of the second fuel component. In some embodiments, the first fuel component comprises about 5, about 10, about 15, about 20, about 25, about 30, about 40, about 50, about 60, about 70, about 75, about 80, about 85, about 90, or about 95 percent by weight of the fuel composition in the upper section. In some embodiments, the remainder of the fuel composition in the upper section 5 may comprise the second fuel component. In some embodiments, the second fuel component comprises about 5, about 10, about 15, about 20, about 25, about 30, about 40, about 50, about 60, about 70, about 75, about 80, about 85, about 90, or about 95 percent by weight of the fuel composition in the lower section. In some embodiments, the remainder of the fuel composition in the lower section 6 may comprise the second fuel component.

As illustrated in Figure 2 the fuel grain 1 may also comprise a latitudinal axis 9. As with the longitudinal axis 3, the fuel grain need not be symmetrically distributed about the latitudinal axis 9. Some fuel grains 1 include a fuel composition that includes first fuel and second fuel components as described herein above. Thus, in some embodiments the viscosity of the fuel composition at a first point 10 is lower than the viscosity of the fuel composition at a second point 11 in the fuel cell. In other embodiments the viscosity at the first point 10 may be higher

HOUDMS/204889.1 than at the second point 11. Fuel compositions having viscosity differences outside these ranges are envisioned. Thus, any fuel grain wherein the viscosity of the fuel composition at a first point in the fuel grain is different than the viscosity of the fuel composition at a second position in the fuel grain is envisioned. Typically, the viscosity of the components should be selected to provide desirable oxidation rates during the planned burn cycle with the understanding that the outer portions of fuel grain 1 are generally consumed before the inner portions.

With continuing reference to Figure 2, some fuel grains 1 include a fuel composition wherein the average molecular weight of the composition at an arbitrary point 10 is lower than the average molecular weight of fuel composition at point 11 with respect to the latitudinal axis 9. In some embodiments, at the first point 10 the fuel composition comprises a first fuel component that includes a paraffinic hydrocarbons having from about 15 to about 100 carbon atoms. In particular embodiments, the first fuel component comprises paraffinic hydrocarbons having from about 18 to about 75 carbon atoms. In other embodiments, the lower limit on the range of carbon atoms of the paraffinic hydrocarbons in the first fuel component is about 20, about 25, about 30, about 35, about 40, or about 50 carbon atoms. In some embodiments, the upper limit on the range of carbon atoms of the paraffinic hydrocarbons in the first fuel component is about 100, about 95, about 90, about 80, about 70, about 60, about 50, or about 40 carbon atoms. In some embodiments, the fuel composition comprises a second fuel component that includes paraffinic hydrocarbons having from about 20 to about 120 carbon atoms. In particular embodiments, the second fuel component comprises paraffinic hydrocarbons having from about 25 to about 100 carbon atoms. In other embodiments, the lower limit on the range of carbon atoms of the paraffinic hydrocarbons in the second fuel component is about 35, about 40, about 50, about 60, about 70, about 80, or about 90 or 95 carbon atoms. In some embodiments, the upper limit on the range of carbon atoms of the paraffinic hydrocarbons in the second fuel component is about 115, about 110, about 100 about 95, about 90, about 80, about 70, about 60, about 50, or about 40 carbon atoms. Regardless of the particular fuel components selected, the fuel composition at the first point 10 with respect to the longitudinal axis preferably has an average molecular weight that is preferably lower than the average molecular weight of the fuel composition at the second point 11.

In some embodiments, the fuel grain 1 further comprises a third fuel component, wherein the relative amount of the third fuel component at the arbitrary first position 10 relative to the

HOUDMS/204889.1 latitudinal axis differs from the relative amount of the third fuel component at the second position 11 relative to the latitudinal axis. The relative amount of the third fuel component is based on the combined weights of the second and third fuel components. In some embodiments wherein the fuel grain includes the third fuel component, the relative amount of the first fuel at the first position 10 relative to the latitudinal axis 3 differs from the relative amount the first fuel component at the second position 11 relative to the latitudinal axis 3; and the relative amount of the first fuel component is based on the combined weights of the first and third fuel components. The third fuel component is generally selected from paraffinic waxes having from about 15 to about 100 carbon atoms.

Figure 3 illustrates an embodiment of the fuel grain 1 wherein the core has an inner section 12 and an outer section 13. In such embodiments, the fuel composition in the outer section 12 is different than the fuel composition of the inner section 13. Typically, the fuel composition of the outer section 12 is selected to burn more readily than the composition of the inner core 13. Thus, the fuel composition of the outer section 12 typically has a lower viscosity than the fuel composition of the inner section 13. In other embodiments, the average molecular weight of the one or more fuel components of the fuel composition of the outer section 13 is lower than the average molecular weight than the one or more fuel components of the inner section 13. The fuel components and ranges that are suitable as fuel compositions are described above. Thus, in some embodiments, the fuel composition of the outer section 12 comprises fuel components having from about 15 to about 100 carbon atoms. In other embodiments, the fuel composition of the outer section 13 comprises fuel components having from about 18 to 75 carbon atoms, 25 to 70 carbon atoms or 30 to about 50 carbon atoms. In some embodiments, the fuel components of the fuel composition of the inner section may have from about 25 to about 120 carbon atoms, about 25 to about 100 carbon atoms, about 35 to 85 carbon atoms, about 45 to75 carbon atoms or about 50 to 60 carbon atoms.

Any suitable paraffinic hydrocarbon can be used as a component of the fuel in the fuel grain. Some suitable paraffinic hydrocarbons can be made by Fischer-Tropsch processes. The Fischer-Tropsch reaction for converting syngas, which is composed primarily of carbon monoxide (CO) and hydrogen gas (H₂) may be characterized by the following general reaction:

2nH₂ + nCO -> (-CH₂-)_n + nH₂O (1)

HOUDMS/204889.1 Non-reactive components, such as nitrogen, may also be included or mixed with the syngas. This may occur in those instances where air or some other non-pure oxygen source is used during the syngas formation.

Three basic techniques may be employed for producing a synthesis gas, or syngas, which is used as the starting material of a Fischer-Tropsch reaction. These include oxidation, reforming and autothermal reforming. As an example, a Fischer-Tropsch conversion system for converting hydrocarbon gases to liquid or solid hydrocarbon products using autothermal reforming includes a synthesis gas unit, which includes a synthesis gas reactor in the form of an autothermal reforming reactor (ATR) containing a reforming catalyst, such as a nickel- containing catalyst. A stream of light hydrocarbons to be converted, which may include natural gas, is introduced into the reactor along with oxygen (O₂). The oxygen may be provided from compressed air or other compressed oxygen-containing gas, or may be a pure oxygen stream. The ATR reaction may be adiabatic, with no heat being added or removed from the reactor other than from the feeds and the heat of reaction. The reaction is carried out under sub-stoichiometric conditions whereby the oxygen/steam/gas mixture is converted to syngas. Examples of Fischer- Tropsch systems are described in U.S. Patent Nos. 4,973,453; 5,733,941; 5,861,441; 6,130,259; 6,169,120; and 6,172,124; the disclosures of which are herein incorporated by reference.

The syngas is delivered to a synthesis unit, which includes a Fischer-Tropsch reactor (FTR) containing a Fischer-Tropsch catalyst. Numerous Fischer-Tropsch catalysts may be used in carrying out the reaction. These include cobalt, iron, ruthenium as well as other Group VIIIB transition metals or combinations of such metals, to prepare both saturated and unsaturated hydrocarbons. For purposes of this invention, a non-iron catalyst may be used. The F-T catalyst may include a support, such as a metal-oxide support, including silica, alumina, silica-alumina or titanium oxides. For the purposes of this reaction, a Co catalyst on transition alumina with a surface area of approximately 100-200 m²/g is used in the form of spheres of 50-150 μm in diameter. The Co concentration on the support may also be 15-30%. Certain catalyst promoters and stabilizers may be used. The stabilizers include Group HA or Group HIB metals, while the promoters may include elements from Group VIII or Group VIIB. The Fischer-Tropsch catalyst and reaction conditions may be selected to be optimal for desired reaction products, such as for hydrocarbons of certain chain lengths or number of carbon atoms. Any of the following reactor configurations may be employed for Fischer-Tropsch synthesis: fixed bed, slurry reactor,

HOUDMS/204889.1 ebullating bed, fluidizing bed, or continuously stirred tank reactor (CSTR). For the purposes of this reaction, a slurry bed reactor is used. The FTR may be operated at a pressure of 100 to 500 psia and a temperature of 190.56 degrees C to 260 degrees C. The reactor gas hourly space velocity ("GHSV") may be from 1000 to 8000 hr^"1. Syngas useful in producing a Fischer- Tropsch product useful in the invention may contain gaseous hydrocarbons, hydrogen, carbon monoxide and nitrogen with H₂/CO ratios from about 1.8 to about 2.4. A detailed description of the Fischer-Tropsch reaction and reaction conditions useful in producing the fuel is contained in co-pending, commonly-owned US Patent entitled "Integrated Improved Fischer-Tropsch Process with Enhanced Oxygenates Processing Capability" and listing Armen Abazajian as inventor, the disclosure of which is incorporated in its entirety herein by reference. The hydrocarbon products derived from the Fischer-Tropsch reaction may range from methane (CH₄) to high molecular weight paraffinic waxes containing more than 100 carbon atoms. Typically, the fuels used in the fuel grains herein have 18 to 100 carbon atoms. Large concentrations of components having fewer numbers of carbons should be avoided where their presence causes the fuel to have an undesirably low melting point.

Some Fischer-Tropsch fuels have from about 5 to about 90 wt. percent linear alpha- and internal olefins, from about 5 to about 20 wt. percent isoparaffins, from about 5 to about 90 wt. percent n-paraffins and from about 0 to about 10 wt. percent oxygenates. While other methods can be used to prepare such fuels, production of the fuel component by the Fischer-Tropsch synthesis and subsequent processing as described herein is especially desirable as it results in a product having the desirable olefin and paraffin contents.

The fuel contains from about 5 to about 90 wt. percent linear alpha- and internal olefins. The olefin content may provide the mixture with lower pour-point, better surface activity, better lubricity and better adherence to metal. When the fuel is produced from the Fischer-Tropsch synthesis with the appropriate Fischer-Tropsch catalyst and operating conditions, the Fischer- Tropsch product will have approximately 5 percent alpha and internal olefin content. Depending upon the reaction conditions of the FTR and catalyst used in the Fischer-Tropsch reaction, it may be necessary to concentrate the olefin content to achieve the higher percentages of olefins in the fuel. Concentration of olefins may be undertaken, for example, by one or more of the following known techniques: (1) molecular sieve separation of olefins and paraffins, such as UOP's Olex process, and (2) distillation of paraffins away from individual C# cuts.

HOUDMS/204889.1 The fuel may contain between about 5 to about 95 wt. percent paraffins. Of the total paraffin content from about 3 to about 20 wt. percent are isoparaffins. Substantially all of the isoparaffins are terminal monomethyl species. For the purposes of this invention, the terminal species are 2- and 3 -methyl branched. The presence of monomethyl isoparaffins improves low temperature properties, such as pour point, as well as lubricity and viscosity. Moreover, because the isoparaffins are predominately terminally branched, the paraffin content of the fuel is substantially wholly biodegradable. Using the Fischer-Tropsch synthesis described herein, about 5 wt. percent terminal methyl branched paraffins are produced in the LFTL.

Concentration of isoparaffins may be increased by one or more of the following techniques: (1) molecular sieve separation of linear and branched paraffins, such as UOP's Molex process, and (2) isomeric distillation of isoparaffin as described in co-pending commonly owned U.S. Application entitled "Hydrocarbon Products and Methods of Preparing Hydrocarbon Products By Skeletal Isomerization of Olefin/Paraffin Mixtures" listing Armen Abazajian as inventor.

The oxygenates are principally primary alcohols. Aldehydes, ketones, carboxylic acids and esters and di-esters of carboxylic acids are present in small amounts. Oxygenate content in fuels of the Fischer-Tropsch reaction product ranges from between about 0.5 to about 5.0 wt. percent. Low levels of oxygenates in the fuel from between about 0 and about 10 wt. percent provides improved lubricity.

Oxygenate control may be used on desired cut of the Fischer-Tropsch product stream. In one embodiment of the invention, a fuel is produced by vapourizing product stream and passing the vaporized product over an activated alumina catalysts to dehydrate alcohols to corresponding olefins. This process is described in detail in the previously incorporated U.S. Application entitled "Integrated Improved Fischer-Tropsch Process with Enhanced Oxygenates Processing Capability." The conversion of the alcohol content of the product stream occurs according to the following reaction:

CH₃-(CH₂)_X-CH₂ - CH₂OH -» CH₃-(CH₂)_X-CH=CH₂ + H₂O (2)

Following dehydration, the aqueous and organic phases may be separated. Such dehydration process may further be used to increase the olefin content of the product stream to be used to produce the fuel.

HOUDMS/204889.1 Other methods of oxygenate control include, for example, reaction of the alcohol content of a Fischer-Tropsch product stream with maleic or succinic anhydride or with a carboxylic acid, such as formic acid, acetic acid, or other acids. The carboxylic acid esters may be retained in the stream as they are excellent lubricants which are also highly biodegradable. Both the lubricity and the biodegradability of the fuel may be improved by converting at least a portion of the alcoholic oxygenate content to carboxylic acid esters.

The fuel may optionally include one or more surfactants (e.g., emulsiflers, wetting agents), viscosifiers, weighting agents, fluid loss control agents, and proppants. Because the fuel should be non-toxic, these optional ingredients, like the fuel, are preferably also non-toxic. Acceptable emulsifiers include, but are not limited to, fatty acids, and fatty acid derivatives including amido-amines, polyamides, polyamines, esters, imidaxiolines, and alcohols. Typical wetting agents include, but are not limited to, lecithin, fatty acids, crude tall oil, oxidized crude tall oil, organic phosphate esters, modified imidazolines, modified amidoamines, alkyl aromatic sulfates, alkyl aromatic sulfonates, and organic esters of polyhydric alcohols. Exemplary weighting agents include, but are not limited to, barite, iron oxide, gelana, siderite, calcium oxide, and calcium carbonate. Acceptable proppants include sand, gravel, and nut shells. Exemplary viscosifiers include, but are not limited to, organophilic clays, non-organophilic clays, oil soluble polymers, polyamide resins, and polycarboxylic acids and soaps. Where additives are used in the fuel, the fuel constitutes from about 25 to about 85 volume percent of the total fuel. Illustrative fluid loss control agents include, but are not limited to, asphaltics (e.g., asphaltenes and sulfonated asphaltenes), modified lignites, and polymers, such as polystyrene, polybutadiene, polyethylene, polypropylene, polybutylene, polyisoprene, natural rubber, and butyl rubber.

Once the fuel components have been selected, they can be formed into a fuel grain by a variety of methods, one such method is described in M. Arif Karabeyoglu et al, "Scale-Up Tests of High Regression Rate Liquifying Hybrid Rocket Fuels", Am. Inst. Aeronautics and Astronautics Inc (2003-64759), incorporated herein by reference in its entirety. As described therein, a centrifugal casting process can be employed. Conditions for the casting process depend on the choice of fuel components, but should be performed to produce fuel grains that have acceptably few void spaces and cracks. Paper phenolic tubes are used as the fuel cartridge. An annular, ATJ graphite insulator is bonded inside each end of tube, using a high-temperature

HOUDMS/204889.1 epoxy. These insulators have a dual purpose. When casting fuel grains, the ATJ insulators mate with Teflon-coated, polyethylene plugs that seal in the contents of the phenolic tube. A blackening agent, typically dye or carbon black, can be to ensure that radiative heat flux into the fuel grain is minimized. The first fuel component and any additives are combined and heated in an 1800 W melting pot. Upon melting, the fuel component is thoroughly mixed and poured into the paper phenolic cartridge.

Depending on the grade of wax used, paraffin waxes can shrink up to 17 percent during solidification. To ensure that the solid fuel grain is void-free, centrifugal casting is employed. Centrifugal casting will also produce a grain that is well bonded to the casing with a single circular port of the desired diameter. Two 0-ring sealed end plates are fitted to the tube allowing it to be mounted on a 2 Hp centrifuge. The centrifuge spins the tube about its axis at high speed (1500 rpm) and the contents of the tube quickly achieve the system's rotational velocity. The first component is spun in the centrifuge until the fuel has solidified. Typically, the fuel may take after several hours to solidify.

After the first fuel component has solidified, the second component is heated and added to the centrifuge. And the centifuging process is repeated. This process can be successively repeated until the each of the desired fuel components have been added. Thus, the fuel grain has a composition that changes depending on the distance from its central axis. Of course, some blending of the individual component may occur at the interface of the successive components. Consequently, the change from one composition to another may be gradual rather than distinct. Once the final fuel component has solidified , the fuel grain is removed from the centrifuge and inspected for defects.

Embodiments of the invention also provide methods for making fuel grains. In some embodiments, the method includes a) providing at least a first fuel component and a second fuel component; and b) forming a core comprising the first and second fuel components, wherein the core has a longitudinal axis and at least one internal aperture therethrough. In some such embodiments, forming the core includes forming a core having a first ratio of the first fuel component to the second fuel component at a first position relative to the longitudinal axis and having a second ratio of the first fuel component to the second fuel component at a second position relative to the longitudinal axis. In some of the methods, the first fuel component has a lower viscosity than the second fuel component. Other methods are characterized by a first fuel

HOUDMS/204889.1 component that has a lower molecular weight than the second fuel component. The fuel components may be any of those fuel components previously described. In particular embodiments, the method provides a wax having less than about 1 ppm of sulfur as at least the first or second fuel component. Some such methods use a wax produced by a Fischer-Tropsch process.

Some methods provide a relative amount of the first fuel component at the first position relative to the longitudinal axis that differs from the relative amount of the first fuel component at the second position relative to the longitudinal axis. In such embodiments, the relative amounts of the fuel components are determined based on the combined weight of the first and second fuel components.

Some methods envisioned by this disclosure further include providing a third fuel component. In such embodiments, forming the core typically includes providing a core having a relative amount of the third fuel component at a first position relative to the longitudinal axis that differs from the relative amount of the third composition at a second position relative to the longitudinal axis. In such embodiments, the relative amount of the third fuel component is based on the combined weights of the first and third fuel components.

In other methods, forming the core includes forming a core having at least an upper section and a lower section wherein the upper section is substantially free of the second fuel component. In still other methods, forming the core comprises forming a core having at least an upper section and a lower section wherein the lower section of the core is substantially free of the first fuel component.

In some methods, the relative amount of the first fuel component a first position relative to the longitudinal axis differs from the relative amount of the first component at a second position relative to the longitudinal axis wherein the relative amount of the first component is based on the combined weights of the first and third fuel components.

In some methods the core has a latitudinal axis and the core includes a relative amount of the first fuel component at a first position relative to the latitudinal axis that differs from the relative amount of the first fuel component at a second position relative to the latitudinal axis; wherein the relative amount of the first fuel component is based on the combined weights of the first and second fuel components. In some such methods, the first fuel component has a lower viscosity than the second fuel component. In other such methods, the first fuel component has a

HOUDMS/204889.1 lower molecular weight than the second foel component. Particularly useful fuel components in such embodiments include waxes having less than about 1 ppm of sulfur, especially those produced by a Fischer-Tropsch process.

Particular methods of making a fuel grain include forming a core comprising a fuel composition having a sulfur content of less than about 1 ppm, wherein the core has a longitudinal axis and at least one aperture therethrough. In some such methods, the fuel composition is a wax prepared by a Fischer-Tropsch processes.

While the invention has been described with a limited number of embodiments, these specific embodiments are not intended to limit the scope of the invention as otherwise described and claimed herein. Moreover, variations and modifications there from exist. For example, various additives, not enumerated herein, may also be used to further enhance one or more properties of the fuel compositions or fuel grains described herein. Embodiments wherein the fuel compositions and/or the fuel grains do not include, or are essentially free of, any components not enumerated herein as envisioned. As used herein the term "essentially free of means that such components are not present in more than trace amounts, such as about 5 or 10 ppm, or are not purposely added to the composition. Also, compositions that consist of or consist essentially of the described components should be considered as disclosed herein.

While the processes are described as comprising one or more steps, these steps may be combined or separated as may be convenient or otherwise desirable. In some embodiments, any step not specifically recited is not included. Other methods consisting the enumerated steps of should be considered as disclosed herein.

Finally, any number disclosed herein should be construed to mean approximate, regardless of whether the word "about" or "approximate" is used in describing the number. Last but not the least, the claimed compositions are not limited to the processes described herein. They can be prepared with any suitable process. The appended claims intend to cover all such variations and modifications as falling within the scope of the invention.

I claim:

HOUDMS/204889.1

Claims

1. A fuel grain comprising: a core having a longitudinal axis and at least one aperture therethrough, wherein the core comprises a fuel composition comprising first and second fuel components, and wherein the core comprises a compositional gradient.

2. The fuel grain according to claim 1, wherein the first fuel component has a lower viscosity than the second fuel component.

3. The fuel grain according to claim 1 , wherein the first fuel component has a lower molecular weight than the second fuel component.

4. The fuel grain according to claim 1, wherein at least the first or second fuel component is a wax having less than about 1 ppm of sulfur.

5. The fuel grain according to claim 1 , wherein the wax is produced by a Fischer-Tropsch process.

6. The fuel grain according to claim 1 , wherein the compositional gradient is characterized by a incremental change in the amount of either the first or the second fuel component.

7. The fuel grain according to claim 1 , wherein the relative amount of the first fuel component at a first position relative to the longitudinal axis differs from the relative amount of the first fuel component at a second position relative to the longitudinal axis; wherein the relative amount of the first fuel component is based on the combined weights of the first and second fuel components.

8. The fuel grain according to claim 1 , wherein the fuel grain further comprises a third fuel component, wherein the relative amount of the third fuel component at a first position relative to the longitudinal axis differs from the relative amount of the third fuel component at a second position relative to the longitudinal axis; wherein the relative amount of the third fuel component is based on the combined weights of the second and third fuel components.

9. The fuel grain according to claim 8, wherein the relative amount of the first fuel at a first position relative to the longitudinal axis differs from the relative amount the first fuel component at a second position relative to the longitudinal axis; wherein the relative amount of the first fuel component is based on the combined weights of the first and third fuel components.

HOUDMS/204889.1

10. The fuel grain according to claim 1 , wherein the fuel grain comprises at least an upper section and a lower section wherein the first fuel component comprises from 0 to about 100 percent by weight of the fuel composition in the upper section; and wherein the lower section comprises from 0 to about 100 percent by weight of the second fuel component.

11. The fuel grain according to claim 1 , wherein the fuel grain comprises a latitudinal axis; and wherein the fuel grain comprises a compositional gradient associated with either the first or second fuel component with respect to the latitudinal axis.

12. The fuel grain according to claim 11, wherein the compositional gradient is characterized by a incremental change in the amount of either the first or the second fuel component.

13. The fuel grain according to claim 1 , wherein the fuel grain comprises a latitudinal axis, wherein the relative amount of the first fuel component at a first position relative to the latitudinal axis differs from the relative amount of the first fuel component at a second position relative to the latitudinal axis; wherein the relative amount of the first fuel component is based on the combined weights of the first and second fuel components.

14. The fuel grain according to claim 11 , wherein the first fuel component comprises from 0 to about 100 percent by weight of the fuel composition at a first position relative to the latitudinal axis; and wherein the second fuel component comprises from 0 to about 100 percent by weight of the fuel composition at a second position relative to the latitudinal axis.

15. The fuel grain according to claim 14, wherein the first fuel component has a lower viscosity or molecular weight than the second fuel component.

16. The fuel grain according to claim 11, wherein the fuel composition further comprises a third fuel component, wherein the relative amount of the third fuel component at a first position relative to the latitudinal axis differs from the relative amount of the third fuel component at a second position relative to the latitudinal axis; wherein the relative amount of the third fuel component is based on the combined weights of the second and third fuel components.

HOUDMS/204₈89.1

17. The fuel grain according to claim 11 , wherein the core comprises at least an inner section and an outer section wherein the relative amount of the first fuel component in the inner section differs from the relative amount of the first fuel component in the outer section.

18. The fuel grain according to claim 17, wherein the fuel grain comprises at least an inner section and an outer section wherein the outer section is substantially free of the first fuel component.

19. The fuel grain according to claim 16, wherein the relative amount of the first fuel component at a first position relative to the latitudinal axis differs from the relative amount of the first fuel component at a second position relative to the latitudinal axis; wherein the relative amount of the first fuel component is based on the combined weight of the first and third fuel components.

20. A fuel grain comprising a core comprising a fuel having less than about 1 ppm of sulfur, wherein the core includes a longitudinal axis and at least one aperture therethrough.

21. The fuel grain according to claim 20, wherein the fuel is selected from waxes produced by a Fischer-Tropsch process.

22. The fuel grain according to claim 21 , wherein the wax is a highly branched wax.

23. The fuel grain according to claim 21 , wherein the wax is a wax prepared by a low- temperature Fischer-Tropsch process.

24. A method of making a fuel grain, comprising: a) providing at least a first fuel component and a second fuel component; and b) forming a core comprising the first and second fuel components, wherein the core has a longitudinal axis and at least one internal aperture therethrough.

25. The method according to claim 24, wherein forming a core includes forming a core having a first ratio of the first fuel component to the second fuel component at a first position relative to the longitudinal axis and having a second ratio of the first fuel component to the second fuel component at a second position relative to the longitudinal axis.

26. The method according to claim 24, wherein the first fuel component has a lower viscosity than the second fuel component.

27. The method according to claim 24, wherein the first fuel component has a lower molecular weight than the second fuel component.

HOUDMS/204889.1

28. The method according to claim 24, wherein at least the first or second fuel component is a wax having less than about 1 ppm of sulfur.

29. The method according to claim 28, wherein the wax is produced by a Fischer-Tropsch process.

30. The method according to claim 25, wherein the relative amount of the first fuel component at the first position relative to the longitudinal axis differs from the relative amount of the first fuel component at the second position relative to the longitudinal axis; wherein the relative amount of the first fuel component is based on the combined weight of the first and second fuel components.

31. The method according to claim 30, wherein the first fuel component has a lower viscosity or molecular weight than the second fuel component.

32. The method according to claim 24, further comprising: providing a third fuel component, wherein forming the core includes providing a core having a relative amount of the third fuel component at a first position relative to the longitudinal axis that differs from the relative amount of the third composition at a second position relative to the longitudinal axis; wherein the relative amount of the third fuel component is based on the combined weights of the first and third fuel components.

33. The method according to claim 24, wherein forming the core comprises forming a core having at least an upper section and a lower section wherein the upper section is substantially free of the second fuel component.

34. The method according to claim 33, wherein forming the core comprises forming a core having at least an upper section and a lower section wherein the lower section of the core is substantially free of the first fuel component.

35. The method according to claim 32, wherein the relative amount of the first fuel component a first position relative to the longitudinal axis differs from the relative amount of the first composition at a second position relative to the longitudinal axis; wherein the relative amount of the first composition is based on the combined weights of the first and third fuel components.

36. The method according to claim 24, wherein the fuel grain comprises a latitudinal axis, and

HOUDMS/204889.1 wherein forming the core includes forming the core having a relative amount of the first fuel component at a first position relative to the latitudinal axis that differs from the relative amount of the first fuel component at a second position relative to the latitudinal axis; wherein the relative amount of the first fuel component is based on the combined weights of the first and second fuel components.

37. The method according to claim 36, wherein the first fuel component has a lower viscosity than the second fuel component.

38. The method according to claim 36, wherein the first fuel component has a lower molecular weight than the second fuel component.

39. The method according to claim 36, wherein at least the first or second fuel component is a wax having less than about 1 ppm of sulfur.

40. The method according to claim 39, wherein the wax is produced by a Fischer-Tropsch process.

41. The method according to claim 36, wherein the fuel composition comprises from 0 to 100 percent by weight amount of the first fuel component at the first position relative to the latitudinal axis ranges, and wherein the fuel composition comprises from 0 to 100 percent by weight of the second fuel component at the second position relative to the latitudinal axis.

42. The method according to claim 41 , wherein the first fuel component has a lower viscosity or molecular weight than the second fuel component.

43. The method according to claim 36, further comprising providing a third fuel component, wherein forming the core includes providing a core having a relative amount of the third fuel component at a first position relative to the latitudinal axis that differs from the relative amount of the third component at a second position relative to the latitudinal axis; and wherein the relative amount of the third fuel component is based on the combined weights of the first and third fuel components.

44. The method according to claim 36, wherein forming the core includes forming a core comprising at least an inner section and a outer section wherein the relative amount of the first fuel component in the inner section differs from the relative amount of the first fuel component in the outer section; wherein the relative amount of the first fuel component is based on the combined weights of the first and second fuel components.

HOUDMS/204889.1

45. The method according to claim 44, wherein the outer section is substantially free of the first fuel component.

46. The method of claim 24, wherein forming the core comprises providing a core comprising at least an inner section and an outer section; and wherein the inner section comprises from 0 to about 100 percent by weight of the first fuel component and the outer section comprises from 0 to about 100 percent by weight of the second fuel component.

47. A method of making a fuel grain, comprising forming a core comprising a fuel composition having a sulfur content of less than about 1 ppm, wherein the core has a longitudinal axis and at least one aperture therethrough.

48. The method of claim 47, wherein the fuel composition is a wax prepared by a Fischer- Tropsch processes.

HOUDMS/204889.1