WO1995011873A1 - High performance middle distillate fuels - Google Patents

Abstract

This invention describes a method of increasing energy density of diesel and jet fuels by introducing extremely high energy density diamondoid components. The diamondoid components comprise a composition which has been processed to remove at least a portion of organics having fewer than 10 carbon atoms, the composition comprising at least about 65 weight percent alkyl-substituted diamondoid compounds which contain more than one quaternary carbon atom per molecule and less than about 35 weight percent of diamondoid compounds which contain less than two quaternary carbon atoms per molecule. The alkyl-substituted diamondoid compounds comprises adamantanes, diamantanes, triamantanes and tetramantanes. The additives of this invention improve the energy density of a middle distillate fuel while maintaining or improving the freeze point and pour points of the resulting mixture. Methods for producing the composition and for operating combustion drivers are also disclosed.

Description

HI6H PERFORMANCE MIDDLE DISTILLATE FUELS

This invention relates to the field of petroleum-based middle distillate fuels. More specifically, this invention provides fuel compositions having improved viscosity stability, enhanced heat capacity per unit volume, and lower cloud point.

Jet fuels are employed in a method of combustion wherein fuel is continuously introduced into and continuously burned in a confined space, for the purpose of deriving power directly from the hot products of combustion. Jet engines typically consist of a propulsion or jet tube, plus a gas turbine which extracts sufficient energy from the departing gases to drive the compressor. In present commercial forms, the compressor and turbine are assembled axially upon a common shaft, spaced far enough apart to permit a number of combustion chambers to be arranged about the shaft between the compressor and turbine, with an exhaust tube extending rearwardly from the turbine. The principal application of such engines is in powering aircraft, particularly for high-altitude operations. For these reasons, the desiderata of fuels useful in jet combustion devices are many and varied.

Jet combustion fuels, as contemplated herein, are hydrocarbon fractions that can have initial boiling points as low as about 200°F (93°C), or lower, and end-boiling points as high as about 600°F (315°C). Depending on the particular application, a jet fuel can boil within a relatively low range of temperatures or within a relatively high range of temperatures. For example, in order to insure quick starting in the operation of military jet- propelled aircraft, jet combustion fuels that boil within rather low temperature ranges are used. These fuels, however, have a high A.P.I. Gravity (or low density), and accordingly, they will have less weight per gallon. As the weight per gallon together with the number of b.t.u. per unit weight is determinative of the amount of energy per gallon of fuel, it is desirable to have the density as high as possible.

In order to improve aircraft performance, the aviation industry in general, and the military in particular, are interested in high density jet fuels which can provide a high volumetric heat of combustion. A new jet fuel, JP-8X, has been proposed, which has an energy density much high than that of the current JP-8 fuel.

Viscosity Index (VI) is the most common measure that is applied to the decrease in viscosity of petroleum oils with increasing temperature. A series of Pennsylvania oils exhibiting relatively small change in viscosity with changing temperature is arbitrarily assigned a VI of 100, whereas a series of Gulf Coast oils whose viscosities change relatively greatly is assigned a VI of 0. From the viscosity measurements at 40° and 100°C, the VI of any oil sample can be obtained from detailed tables published by the ASTM (ASTM D-2270) . 14 Kirk-Othmer Encyclopedia of Chemical Technology 489 (Wiley, 1981) . U.S. Patent 4,913,794 to Le et al. teaches a method for improving the Viscosity Index of a lubricant stock and is incorporated herein by reference for its discussions of Viscosity Index and lubricant upgrading processes. Lubricity and viscosity stability are important characteristics for the fuel compositions of the present invention.

U.S. Patent 5,043,503 to Del Rossi et al. discloses alkylated polycycloparaffinic compounds useful as lubricating stocks which are prepared by alkylating a polycycloparaffinic compound in the presence of a catalyst having a Constraint Index of from about 1 to about 10. U.S. Patent 5,053,568 to Chen teaches a lubricant additive and composition comprising the copolymer of 1- vinyladamantane and a l-alkene having from about 4 to about 16 carbon atoms, wherein the copolymer has a Viscosity Index of at least about 80 and a kinematic viscosity of at least about 6 cS at 212°F. The literature discloses traction fluids containing polycyclic compounds which are characterized by a relatively low Viscosity Index. U.S. Patent 5,085,792 to Narihiko et al. relates to a synthetic traction fluid comprising two substituted cyclohexane nuclei connected through an ester linkage. U.S. Patent 5,107,041 to Abe et al. relates to a synthetic traction fluid derived from a 1,1-dicyclohexyl cycloalkane. For example, the hydrogenated dimer of alpha-methyl-styrene (2,4- dicyclohexyl-2-methylpentane) has a VI of only 9. Many adamantane-based fluids (discussed in U.S. Patents 3,966,624, 3,648,531, 4,043,927, 4,008,251, 3,994,816, and 4,889,649) have high traction coefficients but have very low Viscosity Indices and tend to lose their viscosities quickly as the operating temperatures increase. When the fluid loses its viscosity, it also loses its lubricating film and thus its protection capability. Thus while diamondoid-based mixtures appear to have the potential to contribute high energy density to a middle distillate jet fuel mixture, the prior art suggests that adding such a diamondoid mixture to a middle distillate fuel should reasonably be exected to compromise its temperature stability and freeze point. Summary of the Invention The present invention provides a diamondoid-based composition which is a useful energy density-enhancing additive for a middle distillate fuel such as jet or diesel fuel. The invention further provides a diamondoid- containing middle distillate fuel having enhanced energy density. The additive of the present invention improves the energy density of a middle distillate fuel while maintaining or improving the freeze point and pour points of the resulting mixture. This behavior is surprising and unexpected because it is generally accepted that energy density-enhancing additives generally have low Viscosity Indices, typically within the range of from about -50 to about +50, and high freeze points, and must be diluted with a lighter (lower average molecular weight) solvent to attain the desired low temperature characteristics.

The present invention provides a liquid hydrocarbon combustion fuel comprising a distillate fuel or fuel oil together with a diamondoid-based additive composition which has been processed to remove at least a portion of organics having fewer than 10 carbon atoms, said diamondoid-based composition comprising at least about 65 weight percent alkyl-substituted diamondoid compounds which have more than one quaternary carbon atom per molecule and less than about 35 weight percent of diamondoid compounds which have less than two quaternary carbon atoms per molecule. The hydrocarbon combustion fuel of the invention preferably contains less than about 1 weight percent non-hydrocarbons. Examples of diamondoid compounds which contain more than one quaternary carbon include 1,3-dimethyladamantane; 1,3,5-trimethyladamantane; 1,3,5,7-tetramethyladamantane; cis- and trans-1,3,4-trimethyladamantane; 1,2,5,7- tetramethyladamantane; 4,9-dimethyldiamantane; 1,4- dimethyldiamantane; 2,4-dimethyldiamantane; and 4,8- dimethyldiamantane. The diamondoid-based additive of the invention preferably contains more than 80 weight percent of diamondoid compounds having more than one quaternary carbon atom per molecule and less than 20 weight percent of diamondoids having fewer than two quaternary carbon atoms per molecule.

The diamondoid-based additive composition of the invention may suitably comprise a major portion of diamondoid compounds, for example, at least about 80 weight percent. The additive composition preferably contains at least about 90 weight percent diamondoid compounds and more preferably contains 98 weight percent or more of diamondoid compounds. The fuel oil or distillate fuel component of the diamondoid-containing middle distillate fuel of the invention are hydrocarbon fractions having an initial boiling point of at least about 250°F (121°C) and an end- boiling point no higher than about 750°F (399°C) and boiling substantially continuously throughout their distillation range. Such fuel oils are generally known as distillate fuel oils. It is to be understood, however, that this term is not restricted to straight run distillate fractions. The distillate fuel oils can be straight run distillate fuel oils, catalytically or thermally cracked (including hydrocracked) distillate fuel oils, or mixtures of straight run distillate fuel oils, naphthas and the like, with cracked distillate stocks. Moreover, such fuel oils can be treated in accordance with well-known commercial methods, such as, acid or caustic treatment, hydrogenation, solvent refining, clay treatment, etc.

The distillate fuel oils are characterized by their relatively low viscosities, pour points, and the like. The principal property which characterizes the contemplated hydrocarbons, however, is the distillation range. As mentioned hereinbefore, this range will lie between about 250°F (121°C) and about 750°F (399°C). Obviously, the distillation range of each individual fuel oil will cover a narrower boiling range falling, nevertheless, within the above-specified limits. Likewise, each fuel oil will boil substantially continuously throughout its distillation range.

Contemplated among the fuel oils are Nos. 1, 2 and 3 fuel oils used in heating and as diesel fuel oils, and the jet combustion fuels. The domestic fuel oils generally conform to the specification set forth in A.S.T.M.

Specifications D396-48T. Specifications for diesel fuels are defined in A.S.T.M. Specification D975-48T. Typical jet fuels are defined in Military Specification MIL-F- 5624B. The additive composition of the invention behaves in an unusual and surprising manner at low temperatures. Specifically, the additive composition of the invention can be distilled to remove lower-boiling components without significantly increasing its freeze point but markedly increasing its specific gravity and energy density. This unusual characteristic is of critical importance to its properties as a diamondoid-based additive because previously known high density fuel mixtures required dilution with lower-density hydrocarbons (such as cyclohexane) to maintain an acceptably low freeze point. While adding the lower-density hydrocarbons effectively lowers the freeze point of previously known compositions, the lighter hydrocarbon diluent necessarily decreases the energy density of the mixture. In contrast, mixing the additive composition of the invention with a distillate fuel or fuel oil has been found to maintain or improve the low temperature characteristics of the resulting mixture in comparison with the neat distillate fuel or fuel oil. The additive composition of the invention is generally characterized by a freeze point of less than 0°C, more specifically less than -65°C.

The liquid hydrocarbon combustion fuel of the invention is a useful fuel for combustion drivers including, but not limited to, rocket and gas turbine engines, examples of which include ramjet, turbojet and turboprop engines, as well as internal combustion engines, such as diesel engines.

Non-hydrocarbons may be removed from the hydrocarbon combustion fuel by any suitable step, for example selective sorption, distillation, or stripping. For a survey of adsorptive separation techniques, see "Absorptive Separation" 1 Kirk-Othmer Encyclopedia of Chemical Technology 531-581 (1978) . Nonlimiting examples of useful sorbents include activated carbon and activated alumina. Distillation may also be used to remove non-hydrocarbons and is particularly preferred when the distillation conditions are controlled as disclosed herein to remove both hydrocarbons having fewer than 10 carbon atoms as well as non-hydrocarbons.

Figure 1 is a Simdis chromatograph of the vacuum distilled diamondoid mixture of Examples I and II. The term "Simdis" as used herein refers to a standard gas chromatography procedure described in ASTM D 2887-84, which is entitled "Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography".

Figure 2 is a Simdis chromatograph of the liquid portion of the centrifuged diamondoid mixture of Example III.

Figure 3 is a Simdis chromatograph of the high density fuel mixture of Example IV.

Figure 4 is a Simdis chromatograph of the naturally occurring liquid diamondoid mixture of Example V.

Figure 5 is a Simdis chromatograph of the hydrotreated diamondoid mixture of Example VI.

Figure 6 is a Simdis chromatograph of the diamondoid mixture of Example VII. Figure 7 is a Simdis chromatograph of the diamondoid mixture of Example VIII.

Figure 8 is a Simdis chromatograph of the diamondoid mixture of Example IX.

Figure 9 is a GC trace of the diamondoid mixture of Example X using a capillary column (Supelco Inc., SPB-1, 0.32 ID, 60 M, 1.0 urn) .

Figure 10 is a GC trace of the diamondoid mixture of Example X following the short-path distillation in a Kugelrohr apparatus of Example XI. Figure 11 is a GC trace of the remaining diamondoids produced in Example X after the sample of Example I was purged with a nitrogen stream at room temperature at a rate of 20CC/min.

Figures 12A-12C are the GC traces of the adamantane, diamantane, and triamantane/tetramantane fractions, respectively which were separated in accordance with Example XIV.

Figure 13 shows energy density and freeze point as functions of specific gravity for the compositions of the present invention in the absence of added hydrocarbon containing fewer than 10 carbon atoms.

This invention includes a diamondoid-based additive composition, a liquid hydrocarbon combustion fuel, and a method for operating a combustion-powered driver. The liquid hydrocarbon fuels of the invention exhibit unusual low temperature properties as described above. The additive of the invention is preferably free of non- hydrocarbons including polar sulfur compounds or polar corrosion inhibitors as well as naturally occurring color bodies. These polar compounds may be removed via sorptive media including gamma-alumina, activated charcoal, sand, and clay, merely to name a few. The diamondoid compound- containing additive composition may optionally be hydrotreated to reduce total sulfur and/or to improve color to meet the requirements of the applicable fuel specifications.

The diamondoid-based additive composition of the invention may be further processed (e.g., distilled) to remove lower-boiling diamondoid fractions to increase its energy density while maintaining substantially constant freeze point. Additive Product Conditioning

The diamondoid-based additive composition of the invention may be hydrotreated, but is preferably treated with one or both of two milder conditioning steps. In the present invention, the hydrotreating step is preferably replaced with a simple selective sorption or distillation step. Distillation Process Step The recovered diamondoid compound-containing mixture is charged to a separation stage under vacuum conditions ranging from about 0.05 to about 25 torr (about 66.7 Pa to about 3,330 Pa), preferably from about 0.1 to about 10 torr (about 13.3 Pa to about 1,333.2 Pa), most preferably from about 0.2 to about 2 torr (about 26.7 to about 266.6 Pa). The separation stage is suitably contained in a vessel rated for full vacuum service under the operating temperature selected from the range of about 0°C to about 150"C. The separation stage is preferably connected to a receiver by heated lines of relatively large diameter to avoid condensation or sublimation of valuable products before the products reach the receiver.

The distillation vessel may suitably comprise a flash drum rated for full vacuum service at the endpoint temperature of the diamondoid compound-containing mixture. For example, a vessel equipped with suitable heating means, such as external or internal steam or electric heating coils and a temperature controller would be a useful distillation vessel. The distillation vessel more preferably provides more than one theoretical distillation stage and more preferably provides 10 or more stages in a perforated-plate or packed bed-containing column. The column and associated overhead product recovery conduit are preferably equipped with temperature control to prevent sublimation of solids in the column and overhead product recovery conduit.

To initiate the separation process step in a batch mode embodiment, the separation stage is filled with the crude recovered diamondoid compound-containing mixture and the temperature is raised incrementally and/or pressure is decreased until a vapor product flow is detected, at which point the temperature and pressure are held constant until the vapor product flow ceases. The pot temperature is then raised and/or the pressure is decreased to continue separation. The diamondoid-based additive composition of the invention exhibits unusual freeze point behavior upon distillation. Distilling the composition of the invention to remove lower boiling (less dense, lower molecular weight) fractions increases the energy density and the specific gravity of the mixture without significantly increasing the freeze point of the mixture. Previously known energy density-improving additive compositions compromised energy density to meet a freeze point specification by blending a high energy density component with a low freeze point diluent such as cyclohexane. The diamondoid-based additive composition of the present invention requires no such compromise. Figure 13 shows the energy density and freeze point of an embodiment of the invention as a function of specific gravity. The actual freeze points of the mixtures were well below the stated minimum of the measurement method and are therefore shown as downwardly pointing arrows. This behavior stands in marked contrast to that of previously known energy density- improving additives which require the addition of a low freeze point diluent. Treatment with Sorbent In accordance with the present invention, it has been found that certain diamondoid mixtures may be easily converted into energy density-improving additives by percolation through a suitable medium to sorb impurities such as polar sulfur compounds or polar corrosion inhibitors. Examples of such sorptive media include gamma- alumina, activated charcoal, sand, and clay, merely to name a few. The sorption step may be used in conjunction with the distillation step.

The sorption step may suitably be conducted at ambient temperature and atmospheric pressure, but may also be conducted under elevated conditions of temperature and pressure. For example, suitable temperatures for the sorption step range from about 20 to about 50°C, and suitable pressures range from about atmospheric 14.696 psi (101 kPa) to about 100 psia (689 kPa) . Hydrotreating Processes

The diamondoid-containing additive mixture may optionally be treated under hydrogen pressure in the presence of a hydrotreating catalyst of the type typically used in the petroleum refining industry for desulfurization, denitrogenation, and demetallation of hydrocarbonaceous feedstocks. This step is optional and is not required to meet high density fuel specifications if the sorption and/or distillation steps are conducted as disclosed herein.

Suitable catalysts include metals on an inert or catalytically active support as a heterogeneous catalyst. Useful heterogeneous catalysts may contain metals from Groups IVA or VIIIA of the Periodic Table of the Elements, published by Sargent-Welch of Skokie, Illinois (catalog no. S-18806) . Sulfides and oxides of these metals are also useful catalyst components. Specific examples of useful metals, metallic oxides and sulfides within these groups are exemplified by sulfides and oxides of Ni, Mo, W, Co, Pt, Pd, Cu, and Cr. Bimetallic catalysts including Ni-Mo, Ni-W, Co-Mo, and Cu-Cr are particularly preferred.

Both inert and catalytically active binders may be employed, with examples including one or more of alumina, silica, silica-alumina, zeolites, clays, Kieselguhr, and active carbons from sources such as coal, coke, and coconut shell.

The above-listed metals may also be exchanged onto zeolites to provide a zeolite catalyst having dehydrogenation activity. Suitable zeolites include those commonly referred to as large pore, i.e., those zeolites having a Constraint Index of less than about 1, such as zeolite X, and zeolite Y as well as those commonly referred to as medium-pore, i.e., those zeolites having a Constraint Index of from about 1 to about 12. Examples of suitable medium-pore zeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, and ZSM-48. Zeolite Beta, known to exhibit characteristics of either a large pore zeolite or a medium- pore zeolite, is also useful in the present invention.

The zeolite-containing catalysts useful in the hydrotreating process of the present invention more preferably include those catalysts which exhibit Constraint Indices of from about 0.1 to about 10, for example ZSM-5, MCM-22 and Zeolite Beta, although it is well recognized that the Constraint Index of zeolite Beta varies widely with temperature. Zeolite Beta is described in U.S. Patents 4,696,732; 3,308,069, as well as Re. 28,341, the entire contents of which are incorporated by reference as if set forth at length herein.

A convenient measure of the extent to which a zeolite provides control to molecules of varying sizes to its internal structure is the Constraint Index of the zeolite. The method by which the Constraint Index is determined is described in U.S. Patent Number 4,016,218, incorporated herein by reference for details of the method. U.S. Patent Number 4,696,732, cited above, discloses Constraint Index values for typical zeolite materials and is incorporated by reference as if set forth at length herein for detailed catalyst descriptions and Constraint Index values.

Zeolite ZSM-5 and the conventional preparation thereof are described in U.S. Patent Number 3,702,886, the disclosure of which is incorporated herein by reference. Other preparations for ZSM-5 are described in U.S. Patent Numbers Re. 29,948 (highly siliceous ZSM-5) ; 4,100,262 and 4,139,600, the disclosure of these is incorporated herein by reference. Zeolite ZSM-12 and the conventional preparation thereof are described in U.S. Patent Number

3,832,449, the disclosure of which is incorporated herein by reference.

Catalysts such as ZSM-5 combined with a Group VIII metal described in U.S. Patent No. 3,856,872, incorporated by reference as if set forth at length herein, are also useful in the present invention. Additional catalytic materials useful in the present invention include materials which are readily identified by their characteristic X-ray diffraction patterns, such as the PSH-3 composition of U.S. Patent No. 4,439,409, incorporated herein by reference, and MCM-22, the synthesis and composition of which is taught in U.S. Patent 4,954,325, both of which patents are incorporated by reference as if set forth at length herein.

The zeolite hydrotreating catalyst herein can also be used in intimate combination with a hydrogenating component such as tungsten, vanadium, molybdenum, rhenium, nickel, cobalt, chromium, manganese, or a noble metal such as platinum or palladium where a hydrogenation-dehydrogenation function is to be performed. Such component can be introduced in the catalyst composition by way of cocrystallization, exchanged into the composition to the extent a Group IIIA element, e.g., aluminum, is in the structure, impregnated therein or intimately physically admixed therewith. Such component can be impregnated in, or on, the zeolite such as, for example, by, in the case of platinum, treating the zeolite with a solution containing a platinum metal-containing ion. Thus, suitable platinum compounds for this purpose include chloroplatinic acid, platinous chloride and various compounds containing the platinum amine complex.

The zeolite, especially in its metal, hydrogen and ammonium forms, can be beneficially converted to another form by thermal treatment. This thermal treatment is generally performed by heating one of these forms at a temperature of at least about 370"C for at least 1 minute and generally not longer than 20 hours. While subatmospheric pressure can be employed for the thermal treatment, atmospheric pressure is preferred simply for reasons of convenience. The thermal treatment can be performed at a temperature of up to about 925°C. Prior to its use in the hydrotreating process of this invention, the zeolite crystals should be dehydrated, at least partially. This can be done by heating the crystals to a temperature in the range of from about 200"C to about 595°C in an atmosphere such as air, nitrogen, etc. and at atmospheric, subatmospheric or superatmospheric pressures for between about 30 minutes to about 48 hours. Dehydration can also be performed at room temperature merely by placing the crystalline material in a vacuum. The zeolite crystals can be shaped into a wide variety of particle sizes. Generally speaking, the particles can be in the form of a powder, a granule, or a molded product such as an extrudate having a particle size sufficient to pass through a 2 mesh (Tyler) screen and be retained on a 400 mesh (Tyler) screen. In cases where the catalyst is molded, such as by extrusion, the crystals can be extruded before drying or partially dried and then extruded.

It may be desired to incorporate the crystalline material with another material which is resistant to the temperatures and other conditions employed in the hydrotreating process of this invention. Such materials include active and inactive materials and synthetic or naturally occurring zeolites as well as inorganic materials such as clays, silica and/or metal oxides such as alumina. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Use of a material in conjunction with the zeolite, i.e., combined therewith or present during its synthesis, which itself is catalytically active may change the conversion and/or selectivity of the catalyst. Inactive materials suitably serve as diluents to control the amount of conversion so that hydrotreated products can be obtained economically and orderly without employing other means for controlling the rate of reaction. These materials may be incorporated into naturally occurring clays, e.g., bentonite and kaolin. Said materials, i.e., clays, oxides, etc., function as binders for the catalyst. These clay binders have been employed normally only for the purpose of improving the crush strength of the catalyst. Naturally occurring clays which can be composited with zeolite crystals include the montmorillonite and kaolin family, which families include the subbentonites, and the kaolins commonly known as Dixie, McNamee, Georgia and Florida clays or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite, or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. Binders useful for compositing with the zeolite also include inorganic oxides, notably alumina.

In addition to the foregoing materials, the crystals can be composited with a porous matrix material such as silica-alumina, silica-magnesia, silica-zirconia, silica- thoria, silica-beryllia, silica-titania as well as ternary compositions such as silica-alumina-thoria, silica-alumina- zirconia silica-alumina-magnesia and silica-magnesia- zirconia. It may also be advantageous to provide at least a part of the foregoing matrix materials in colloidal form so as to facilitate extrusion of the bound catalyst component(s) .

The relative proportions of finely divided crystalline material and inorganic oxide matrix vary widely, with the crystal content ranging from about 1 to about 90 percent by weight and more usually, particularly when the composite is prepared in the form of beads, in the range of about 2 to about 80 weight percent of the composite.

The stability of the hydrotreating catalyst of the invention may be increased by steaming. U.S. Patent Nos. 4,663,492; 4,594,146; 4,522,929; and 4,429,176, the entire disclosures of which are incorporated herein by reference, describe conditions for the steam stabilization of zeolite catalysts which can be utilized to steam-stabilize the catalyst.

The optional hydrotreating step may suitably be conducted under the conditions listed below in Table 1. TABLE 1

Hydrotreating Reaction Conditions

Process Variable Broad Preferred

Temperature, °C 30-220 50-180

WHSV, based on catalyst. 0.01-100 0.1-5 Pressure, kPa 100-14,000 170-7000 Hydrogen Dosage, Mols H₂/Mol diamondoid mixture 0.05-100 1-10

Example I A mixture of diamondoid compounds recovered from a diamondoid-containing natural gas well was separated from a solvent by vacuum distillation. The mixture contained 10- 15% by volume of a white crystalline solid which was rendered into a homogeneous liquid by the addition of hexane. The hexane solution of the diamondoids was then percolated through activated alumina. After removal of the hexane, the mixture of diamondoid compounds was cooled in a dry ice/acetone bath for 24 hours and then transferred into a glove bag using a pre-cooled Buchner funnel. The temperature of the mixture in the Buchner funnel was -60°C at the beginning of the filtration and -40"C at the end of the filtration. Fuel properties of the liquid filtrate are shown in Table 2. Figure 1 shows a Simdis chromatograph (ASTM Designation D 2887-84) of the vacuum distilled diamondoid mixture of Examples I and II and Table 3 summarizes the Simdis boiling range of the mixture.

Example II The fuel described in Example 1 was hydrogenated over Ni on Kieselguhr at 230"C overnight. The product was filtered and evaluated as high density fuel. The sulfur content was reduced to 0.002% by weight while the other properties remained unchanged, as summarized in Table 2. Figure 1 shows a Simdis chromatograph (ASTM Designation D 2887-84) of the vacuum distilled diamondoid mixture of Examples I and II and Table 3 summarizes the Simdis boiling range of the mixture.

Example III The mixture of diamondoid compounds as used in Example 1 was equilibrated in a centrifuge maintained at a constant temperature of -20°C for 72 hours, and the solid part of the mixture was centrifuged off. The liquid was found to have an API gravity of 20.9, a sulfur content of 0.096% by weight, and a net heat of combustion of 44.2 megajoules per kilogram, or 41.0 megajoules per liter. These results are summarized below in Table 2. Figure 2 shows a Simdis chromatograph of the liquid portion of the centrifuged diamondoid mixture of Example III and Table 3 summarizes the Simdis boiling range of the mixture.

Example IV The mixture of diamondoid compounds as used in Example 1 was distilled without a column under vacuum. The first fraction which came over as a solid at 25-40"C and 0.5 torr was essentially adamantane. The second fraction which came over as a colorless liquid at 75-125°C and 0.5 torr was a complex mixture of essentially methylated adamantanes. The third fraction which came over as a solid at >125°C and 0.5 torr contained essentially diamantane. The colorless liquid fraction was evaluated as high density fuel, and was found to have a freezing point of <50°C, an API gravity of 22.5, a sulfur content of 0.076% by weight, and a net heat of combustion of 42.4 megajoules per kilogram or 39.1 megajoules per liter. These results are shown below in Table 2. Figure 3 shows a Simdis chromatograph of the high density fuel mixture of Example IV and Table 3 summarizes the Simdis boiling range of the mixture. Example V A mixture of diamondoid compounds was recovered from a diamondoid-containing natural gas well as a liquid. The mixture of diamondoid compounds was tan in color and contained solid contaminants. The mixture was percolated through freshly calcined high activity gamma-alumina at room temperature. The mixture became water-white, and was evaluated as high density fuel without further treatment. The treated mixture was found to have a freezing point of <-80°C, an API gravity of 18.8, a sulfur content of 0.004% by weight, and a net heat of combustion of 42.2 megajoules per kilogram or 39.8 megajoules per liter. These results are shown below in Table 2. Figure 4 shows a Simdis chromatograph of the naturally occurring liquid diamondoid mixture of Example V and Table 3 summarizes the Simdis boiling range of the mixture. The sample of Example V contained about 78 weight percent of diamondoid compounds having more than one quaternary carbon atom per molecule.

Example VI A mixture of diamondoid compounds was recovered from a diamondoid-containing natural gas well and separated from a solvent by distillation. The separated diamondoid mixture was found to contain 0.37% sulfur by weight. The mixture was then hydrotreated over a commercial Co-Mo/Al₂0₃ catalyst. The treated diamondoid mixture was evaluated as a high density fuel without further modification. The mixture contained, at room temperature, approximately 10- 15% by volume of a white crystalline solid at the bottom and the rest was a clear liquid. The solid components dissolved into the liquid at «60°C, although the mixture contain solids, it was readily pourable at low temperatures. The pour point of the mixture was determined to be -4°C. Missile fuel properties of this mixture are shown in Table 2. Figure 5 shows a Simdis chromatograph of the hydrotreated diamondoid mixture of Example VI and Table 3 summarizes the Simdis boiling range of the mixture. The sample of Example VI contained about 36 weight percent of diamondoid compounds having more than one quaternary carbon atom per molecule.

Example VII A mixture of diamondoid compounds containing 75% by weight of the diamondoid mixture of Example I and separated from a solvent by vacuum distillation used in Example VI, and 25% by weight of the diamondoid mixture of Example V was made and evaluated as a high density fuel. This mixture contained approximately 10% by volume of white crystalline solids at room temperature and had a pour point of -114°C. Fuel properties of this mixture are shown in Table 2. Figure 6 shows a Simdis chromatograph of the diamondoid mixture of Example VII and Table 3 summarizes the Simdis boiling range of the mixture. The sample of

Example VII contained about 39 weight percent of diamondoid compounds having more than one quaternary carbon atom per molecule.

Example VIII A mixture of diamondoid compounds contained 50% by weight of the diamondoid mixture used in Example VI, and 50% by weight of the diamondoid mixture used in Example V was made and evaluated as a high density fuel. The mixture contained approximately 5% by volume of a white crystalline solid at room temperature and had a pour point of -120°C. Missile fuel properties of this mixture are shown in Table 2. Figure 7 shows a Simdis chromatograph of the diamondoid mixture of Example VIII and Table 3 summarizes the Simdis boiling range of the mixture. The sample of Example VIII contained about 52 weight percent of diamondoid compounds having more than one quaternary carbon atom per molecule.

Example IX A mixture of diamondoid compounds containing 25% by weight of the diamondoid mixture used in Example VI, and 75% by weight of the diamondoid mixture in Example V, was evaluated as a high density fuel. This mixture was clear at room temperature and had a pour point of -105°C. The properties of this mixture are shown in Table 2. Figure 8 shows a Simdis chromatograph of the diamondoid mixture of Example IX and Table 3 summarizes the Simdis boiling range of the mixture. The sample of Example IX contained about 65 weight percent of diamondoid compounds having more than one quaternary carbon atom per molecule.

Table 2, below, shows the properties of the mixtures of Examples I-IX. The properties of a jet fuel blending component designated as "Jet A" are presented for comparison.

Table 2 Properties of Diamondoid Mixtures

Jet Ex. I Ex. II Ex. Ill Ex. IV EX. V Ex. VI Ex. VII Ex. VIII Ex. I Fuel "A"

Flash Point, °C 70 70 — — — — — — —

Freeze Point, °c -40.5 <-40 <-40 — <-50 <-50

Pour Point, °C -4 -114 -120 -105

Density (15°C) kg/m³ 805.1 926.3 926.3 927.6 917.9 940.5 989.5 963.2 961.9 955.

Viscosity, cS at 4.0 4.0 — — — — — — — 37.8°C

Net Heat of Combustion

MJ/Kg 43.9 42.4 42.4 42.2 42.4 42.3 42.3 42.3 42.

MJ/L 40.7 40.7 39.4 39.1 39.8 42.0 40.8 40.7 40.

BTU/lb 18953 18861.6 18861.6 18242.0 18242.5 18160.0 18242.5 18201.2 18201.2 18201.

BTU/gallon 127107 145,537 145,537 140,452 139,492 142,272 150,364 146,037 153,519 144,8

Sulfur, Total (wt%) 0.080 0.096 0.076 0.004 0.002 0.002 0.003 0.0

0.086 0.002

TABLE 3

SUMMARY OF Simdis BOILING RANGE OF HIGH DENSITY FUELS

B. P. ° Έ

E X A M P L E wt. % I II III IV V VI VII VIII IX

10 368 368 373 373 370 372 372 372 366

20 375 375 377 378 382 376 377 378 376

30 379 379 381 382 390 380 382 383 383

40 382 382 384 385 410 384 386 388 396

50 385 385 387 388 423 388 400 413 413

60 388 388 392 392 446 421 427 435 431

70 395 395 401 396 500 503 502 503 476

80 423 423 429 422 522 513 513 515 513

90 507 507 509 502 543 530 530 539 535

99 546 546 542 528 645 911 656 703 644

100 614 614 586 543 695 1062 793 926 702

Notably, the diamondoid-containing mixtures produced by the process of the present invention exhibit higher energy densities than required for military jet fuel. Example X

A normally liquid diamondoid sample was transferred into a clear container with a faucet at the bottom. The natural gas well which produced this normally liquid diamondoid sample required no solvent injection and the normally liquid diamonoid sample was recovered neat. The aqueous and oil phases were clearly separated in this sample. The aqueous phase was drained from the container and discarded. The oil phase, which was colored pale yellow, was percolated through a column of activated alumina (EM Science No. AX 0612-3, chromatographic grade, 80-200 mesh, ALCOA Type F-20) to a clear water-white liquid. The composition and properties of this clear mixture are given in Column 4 of Table 2. Figure 9 is a GC trace of the mixture using a capillary Column (Supelco Inc., SPB-1, 0.32 ID, 60 M, 1.0 urn). The mixture was found to contain 6.5% of hydrocarbons boiling lower than adamantane and a good deal of these pre-adamantane materials were extremely low-boiling. The low flash point was attributed to the very low boiling components. This diamondoid mixture was also found to contain 481 mg/liter extremely high boiling material which was initially classified as "existent gum", but which, upon further analysis, was found to contain high molecular weight diamondoids such as pentamantane. The mixture had a high net heat of combustion per volume (146,894 Btu/gallon) and also had good low temperature properties. The properties of this mixture are summarized below in Table 4 together with JP-10 and RJ-5 missile fuel specifications for comparison.

TABLE 4 Missile Fuel Specifications and Properties of Diamondoid Mixtures

1 2 3 4

JP-10 RJ-! 5 RJ- ^■6 Example X

Property Min Max Min Max Min Max Diamondoid Mixtur

Color, Saybolt +25 +25 +25 +30

Chem. comp. , wt%

Exo-tetrahydroidi-

(cyclopentadiene) 98.5 100 —__ —.__ 36 42

Perhydroi-(norboradiene) 98.5 100 58.0 64.0

Other hydrocarbons 1.5 1.5 1.5

Pre-adamantanes 6.5

Adamantanes 45.6

Diamantanes 40.2

Triamantanes + ——— ——— ——— ——— ——— 7.8

Tetramantane Flash point 54.4 95 60 -15.6 Sp. gr., (60/60F) 0.935 0.943 1.060 1.010 1.050 0.9799 Freezing point C -79 -54 <-65 Pour point C ——— -30 <-53.9 Viscosity, cS at -54 C 40 400 593 at -18 C 10 400 40 39.7 Net heat of combustion

Btu/lb 18,100 17,750 17,850 17,996

Btu/gallon 141,500 160,000 152,1 300 146,894 Existent gum mg/liter 1.0 100 100 Particulate matter, mg/liter 1.0 1.0 1.0 1.0

Example XI The clear mixture obtained in Example X was subjected to a short-path distillation in a Kugelrohr apparatus. The total mixture was distilled at a pot temperature of 150"C and a vacuum of 4.0 torr. The low boilers were collected in a dry ice/acetone trap which was placed after the receiver (cooled by being partially immersed into a dry ice/acetone bath) for the diamondoids. Some residue (pentamantanes) remained in the pot. The composition and properties of the resulting diamondoids are given in Column 1 of Table 5. The GC results of the distilled mixture is given in Figure 10, and the composition and properties of the distilled mixture are also shown in Table 5. It is indicated that both the low boilers and the residue were removed by this simple operation. The flash point was raised to 82.2°C. As shown in Figure 10, the pentamantane peak disappeared in the GC analysis. The heat of combustion was found to be higher than in Example X due to the removal of the low density low boilers. Surprisingly, the low temperature properties remain good with the exception that the viscosity at -54°C was high.

Example XII One liter of the clear diamondoids obtained in Example X was purged with a nitrogen stream at room temperature at a rate of 20CC/min. The nitrogen outlet was passed through a dry ice/acetone cold trap. After an hour, the operation was stopped. Most, but not all, of the low boilers were removed into the cold trap. The GC results of the remaining diamondoids are shown in Figure 11. The properties and the compositions are given in Column 2 of Table 5. The flash point was raised considerably to 62.8°C. TABLE 5

Example XI Example XII Short-Path Nitrogen

Propertv Distillation Stripping Color, Saybolt 30 30 Chem. comp. , wt% 0.6 2.7 Adamantanes 48.5 48.9 Diamantanes 43.5 41.4 Triamantanes + 7.4 7.0 Tetramantanes Flash Point, °C 82.2 62.8 Sp. gr. 0.9868 0.9826

Freezing pt. , °C <-62.5 <-65.0 Pour pt. , "C <-53.9 <-53.9 Viscosity, cS at -54°C 2963 1865 at -18°C 94.6 74.2 Net heat of combustion Btu/lb 18,026 18,060 Btu/gallon 148,174 149,358

Existent gum mg/liter 98 247 Particulate matter

Example XIII A portion of the crude diamondoid sample of Example X was transferred from storage to a clear container with a faucet at the bottom. The aqueous phase was drained. The oil phase was dried over anhydrous sodium sulfate. The dried oil phase was subjected to a short-path distillation at a pot temperature of 150"C, a vacuum of 5 torr. The low boilers were collected in a dry ice/acetone trap, and the diamondoids were collected in the receiver at room temperature. Essentially, the same results were obtained as in Example XI. This example shows that the process of the present invention can readily convert a naturally occurring mixture of diamondoids to a high density fuel by one simple short-path distillation, even without first percolating through activated alumina as described in Example X. Further, the process does not require impractically high vacuum, nor does it require expensive refrigeration of the receiver for the diamondoids. Example XIV

The diamondoids cleaned up as described in Example X were fractionated into (1) low boilers, (2) adamantanes, (3) diamantanes, (4) triamantanes + tetramantanes + pentamantanes. The composition and properties of each fraction are given in Table 6. Figure 12 shows the GC results of the fractions.

TABLE 6

Chemical and Physical Properties of

Crude Diamondoid Mixture and Fractions

Example X Crude Example XIV

Diamondoid Fraction Fraction Fraction Fraction Mixture (1) (2) (3) (4)

Saybolt Color +30 +30 +30 +30 +30

Chem. comp. wt% Light Hydroc. 6.5 100 0.7 0 0 Adamantanes 45.6 0 98.5 3.3 0 Diamantanes 40.2 0 0.8 95.2 0.6 Triamantanes 7.8 0 0 1.5 99.4 + Tetramantanes

Flash point, °C -15.6 ... — — 80

Sp. gr. 0.9799 0.8309 0.9224 1.0246 1.0901

Freezing pt.,^cC <-65 <-65 <-65 <-53.9* -51

Pour pt.,°C <-53.9 <-53.9 <-53.9 <-53.9 -51

Viscosity, cS at -54°C 593 180.00 14,8332 > 99,999 at -18°C 39.7 — 21.6 267.5 4,538

Net heat of combustion

Btu/lb 17,999 17,901 18,156 17,883 17,454

Btu/gallon 146,894 23,770 139,500 152,629 158,500

Existent gum mg/liter Particulate matter, mg/liter 1.0

Sulfur, ppm 0.006 < 0.002 < 0.002 0.004 0.018

Too viscous to be i a accttuuaallllyy ddeetteerrmmined below this temperature.

Examples XV - XVIII Examples XV-XVIII show that the diamondoid-containing fractions produced in accordance with the present invention can be blended to formulate useful energy density-enhancing additives. Generally, the fraction containing triamantanes and tetramantanes was found to have the highest density and the highest heat of combustion per volume. The following Examples show that the desirable qualities of high heat density can be combined with good low temperature properties by blending the fraction containing triamantanes and tetramantanes with single compounds such as cyclohexane.

Example XV A blend was made by mixing 75 wt% of the diamondoid mixture of Example X and 25 wt% of Fraction (3) prepared in Example XIV. The composition and properties are shown in Column 1 of Table 7. The addition of this amount of diamantanes to the total mixture increased the heat of combustion per volume, and retained the good low temperature properties.

Example XVI A blend was made by mixing 50 wt% of the diamondoid mixture of Example X and 50 wt% of Fraction (3) of Example XIV. The composition and properties are shown in Column 2 of Table 7.

Example XVII A blend was made by mixing 92.5 wt% of the diamondoid mixture of Example XIII and 7.5 wt% of cyclohexane. Results are shown in Table 8.

Example XVIII A blend was made by mixing 45 wt% of the diamondoid mixture of Example I, purified as described in Example IV, 45 wt% of Fraction (3) , the diamantanes, prepared in Example V, and 10 wt% of cyclohexane. Results are shown in Table 8. TABLE 7 Properties of Diamondoid Blends

Example XV Example XVI

Property Mixture Mixture

Color, Saybolt +30 +30

Chem. comp. wt%

Light Hydroc. 4.4 3.0

Adamantanes 34.3 23.9

Diamantanes 55.9 69.1

Triamantanes + 5.4 4.0

Tetramantanes

Flash point, °C

Sp. gr. 0.98847 0.9979

Freezing pt. , °C <-65°C <-62°C

Pour pt. , "C <-53.9 <-53.9

Viscosity, cS at -54"C at -18°C

Net heat of combustion

Btu/lb 18,063 18,030

Btu/gallon 148,165 149,872

Existent gum mg/liter

Particulate matter mg/liter

Sulfur, ppm 0.006 0.004

TABLE 8 Properties of Diamondoid Blends

Example XVII Example XVIII

Property Mixture Mixture

Color, Saybolt +30 +30

Chem. comp. wt% Pre-adamantanes 9.2 8.9 Adamantanes 41.9 19.2 Diamantanes 42.0 67.5 Triamantanes 6.9 4.4 and higher

Flash point, °C 92.2 90.0

Sp. gr., (60/60°F) 0.9692 0.9799

Freezing pt. , °C <-65 <-65

Pour pt. , °C <-53.9 <-53.9

Viscosity, cS at -54°C 1059 1756 at -18°C 52.8 66.7

Net heat of combustion Btu/lb 18,068 18,072 Btu/gallon 145,872 147,514

Sulfur, Wt% 0.004 0.004

Example XIX The diamondoids cleaned up as described in Example X were fractionally distilled to remove the fractions of low boilers and adamantanes, leaving the diamantanes, triamantanes and higher homologues in the pot. The pale- yellow liquid in the pot was percolated through activated alumina to obtain a water-white material of excellent missile fuel properties. The composition and properties of this material is given in Column 1 of Table 9.

Example XX A mixture was made of 90 wt % of the diamondoids obtained in Example XIX and 10 wt % of cyclohexane. The composition and properties are shown in Column 2 of Table 9. The low temperature viscosities have been lowered from Example XIX by the addition of cyclohexane.

TABLE 9 Properties of Examples XIX and XX

Property Example XIX Example XX

Color, Saybolt +30 +30

Che . comp. wt%

Light Hydroc. 0 11.8

Adamantanes 8.0 7.0

Diamantanes 78.7 69.0

Triamantanes 13.3 12.1

+ Tetramantanes

+ Pentamantanes

Flash point, "C 123.9 102.3

Sp. gr., (60/60°F) 1.055 0.9993

Freezing pt. , °C <-50 <-60

Pour pt. , "C <-53.9 <-53.8

Viscosity, cS at -54°C 35,627 5762 at -18°C 418.7 138.7

Net heat of combustion

Btu/lb 17,913 17,988

Btu/gallon 157,419 149,731

Sulfur, ppm 0.002 0.002

Example XXI and XXII A mixture was made of 50 wt % of diamondoid Fraction 3 obtained in Example XIV and 50 wt % of a commercially available jet fuel Jet A which is used to formulate JP-8. The composition and properties of the commercial jet fuel are shown in Column 1 of Table 10 as Example XXI, while the composition and properties of the diamondoid-enhanced jet fuel mixture are shown in Column 2 of Table 10 as Example XXII. TABLE 10 Properties of Examples XXI and XXII

Property Example XXI Example XXII

Color, Saybolt +30 +30

Composition, wt.%

Jet A 100 50 Fraction 3 of

Example XIV 0 50

Sp. gr., (60/60°F) 0.8051 0.9238

Freezing pt. , °C -40.5 -41.5

Net heat of combustion Btu/lb 18,953 18,342 Btu/gallon 127,107 141,146

Sulfur, ppm 0.080 0.038

Examples XXIII and XXIV

A No. 6 diesel fuel was injected into the surface equipment of an operating natural gas field to solve the diamondoid plugging problem according to U.S. 4,982,049. After the diesel fuel was saturated with the diamondoids, the diamondoid-saturated mixture was withdrawn from the surface equipment and percolated through activated alumina to remove any corrosion inhibitors used to prevent corrosion of the equipment. The net heat of combustion of the cleaned up diamondoid saturated No. 6 diesel fuel was found to have increased by 5,439 Btu/Gallon from the injected No. 6 diesel fuel while the low temperature properties were also improved. The properties of the No. 6 diesel are shown in Table 11 as Example XXIII, while the properties of the diamondoid saturated No. 6 diesel are shown in Table 11 as Example XXIV. TABLE 11 Properties of Examples XXIII and XXIV

Property Example XXIII Example XXIV

Net heat of combustion Btu/lb 18,623 18,062 Btu/gallon 131,208 136,647

API Gravity 35.8 24.3

Sp. Gravity 0.8458 0.9082

Sulfur, % 0.5 0.46

Freezing pt. , °C -4.0C -16.5C

Viscosity, cS at -18 C N/A, Solid 23.9 at -54 C N/A, Solid N/A, Solid

Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.

Claims

We claim:

1. A hydrocarbon combustion fuel comprising:

(a) a hydrocarbon fraction having an initial boiling point of at least about 250°F (about 121°C) and an end-boiling point no higher than about 750°F (about 399°C), and boiling substantially continuously throughout the range of from about 250°F (about 121°C) to about 750°F (about 399°C); and

(b) a diamondoid-based additive composition which has been processed to remove at least a portion of organics having fewer than 10 carbon atoms, said composition comprising at least about 65 weight percent alkyl- substituted diamondoid compounds which contain more than one quaternary carbon atom per molecule and less than about 35 weight percent of diamondoid compounds which contain less than two quaternary carbon atoms per molecule.

2. The composition of claim 1 wherein said diamondoid-based additive comprises less than 1 weight percent of non-hydrocarbons.

3. The composition of claim 1 further characterized by a freeze point of less than about 0°C in the absence of added C₉- hydrocarbons.

4. The composition of claim 3 further characterized by a freeze point of less than -65"C in the absence of added C₉- hydrocarbons.

5. The composition of claim 4 further characterized by a flash point of more than +50°C.

6. The composition of claim 5 further characterized by a flash point of more than +60°C.

7. The composition of claim 1 which has been processed to a Saybolt color of at least about 25.

8. The composition of claim 7 which has been processed to a Saybolt color of about 30.

9. The composition of claim 8 wherein said composition is water white as defined herein.

10. A hydrocarbon combustion fuel comprising:

(a) a hydrocarbon fraction having an initial boiling point of at least about 250°F (about 121°C) and an end-boiling point no higher than about 750°F (about 399°C) , and boiling substantially continuously throughout the range of from about 250°F (about 121°C) to about 750°F (about 399°C) ; and

(b) a diamondoid-based additive composition comprising at least about 90 weight percent diamondoid compounds which composition has been processed to remove at least a portion of organics having fewer than 10 carbon atoms, said composition comprising at least about 65 weight percent alkyl-substituted diamondoid compounds which contain more than one quaternary carbon atom per molecule and less than about 35 weight percent of diamondoid compounds which contain less than two quaternary carbon atoms per molecule.

11. The composition of claim 10 consisting essentially of diamondoid compounds.

12. The composition of claim 10 further characterized by a freeze point of less than 0°C in the absence of added hydrocarbons having fewer than 10 carbon atoms.

13. The composition of claim 12 further characterized by a freeze point of less than -65"C in the absence of added hydrocarbons having fewer than 10 carbon atoms.

14. The composition of claim 10 further characterized by a flash point of more than +50°C.

15. The composition of claim 10 which has been processed to a Saybolt color of at least about 25.

16. A method for operating a combustion driver comprising charging to said driver a fuel composition comprising:

(a) a hydrocarbon fraction having an initial boiling point of at least about 250°F (about 121°C) and an end-boiling point no higher than about 750°F (about 399°C) , and boiling substantially continuously throughout the range of from about 250°F (about 121°C) to about 750°F (about 121°C); and (b) a diamondoid-based additive composition which has been processed to remove at least a portion of organics having fewer than 10 carbon atoms, said composition comprising at least about 65 weight percent alkyl-substituted diamondoid compounds which contain more than one quaternary carbon atom per molecule and less than about 35 weight percent of diamondoid compounds which contain less than two quaternary carbon atoms per molecule.

17. The method of claim 16 wherein said driver is selected from the group consisting of rocket, ramjet, turbojet, and turboprop engines.

18. A process for making a hydrocarbon combustion fuel comprising the steps of:

(a) providing a natural gas stream having a diamondoid compound-containing mixture dissolved therein;

(b) recovering said diamondoid compound-containing mixture;

(c) stripping hydrocarbons having less than 10 carbon atoms from said recovered diamondoid compound- containing mixture with an inert stripping gas at about ambient temperature;

(d) recovering a diamondoid-based additive composition comprising at least about 65 weight percent alkyl-substituted diamondoid compounds which contain more than one quaternary carbon atom per molecule and less than about 35 weight percent of diamondoid compounds which contain less than two quaternary carbon atoms per molecule; and (e) admixing said diamondoid-based additive composition with a hydrocarbon fraction having an initial boiling point of at least about 250°F (about 121°C) and an end-boiling point no higher than about 750°F (about 399°C), and boiling substantially continuously throughout the range of from about 250°F (about 121°C) to about 750°F (about 399°C) .

19. The method of claim 18 further comprising removing non-hydrocarbons from said recovered diamondoid- containing mixture of step (b) or said stripped diamondoid- containing mixture of step (c) .