Hydrocarbon fuel with improved laminar burning velocity and method of making

Abstract

A hydrocarbon fuel such as a gasoline exhibiting substantially improved laminar burning velocity and method of making. The hydrocarbon fuel may comprise a paraffinic fraction, an olefinic fraction, and an aromatics fraction. The aromatics fraction may comprise methyl aromatics and non-methyl alkyl aromatics wherein the percentage of non-methyl alkyl aromatics in the aromatics fraction is at least 30% by volume. The fuel may comprise a methyl aromatics fraction comprising xylenes wherein the percentage of ortho- and para-xylene in the xylene fraction is at least 60% by volume.

Description

This application claims the benefit of U.S. Ser. No. 60/485,001 filed Jul. 3, 2003.

FIELD OF THE INVENTION

The present invention relates to an improved. hydrocarbon fuel and method for making it. More specifically, it relates to a hydrocarbon fuel exhibiting improved laminar burning velocity. The improved hydrocarbon fuel substantially increases engine efficiency.

BACKGROUND OF THE INVENTION

Increasingly more stringent emissions and efficiency regulations pose a significant hurdle to internal combustion engine makers. Current spark ignition and compression ignition engine efficiencies are well below the theoretical maxima, and even small efficiency improvements are highly desirable. Many engine makers are developing sophisticated hardware controls to extract more efficiency from the combustion cycle. For example, techniques such as direct injection, homogeneous charge compression ignition, variable valve timing, and turbocharging have been commercialized to varying levels, and have proved successful in improving efficiency. The effects of fuel composition on engine efficiency have also been actively studied. Presently, a fuel's octane number is considered to have the most significant impact on engine efficiency, since higher octane number fuels allow a closer approach to optimum spark advance timing and permit increased compression ratio operation. The effects of the fuel's laminar burning velocity (or the closely related laminar flame speed) on engine efficiency have also been studied but are not as well understood. It is generally recognized that faster burn rates in engines lead to higher efficiency. For this reason there has been a trend in engine designs in recent years to modify the mechanical design of the fuel system and/or combustion chamber (e.g., increased swirl and/or tumble) to enhance burn rates. Engine correlation tools developed to predict burn rates traditionally incorporate the fuel's laminar flame speed (SAE800133). Further, it has been shown that increases in engine burn rates in a modern lean burn type engine correlate directly with increases in fuel laminar flame speed measurements made in a constant volume combustion chamber (U.S. Pat. No. 6,206,940). However, laminar flame speeds or burning velocities of fully blended fuels are not typically measured, nor are they readily estimated through surrogate analytical techniques. Whereas standardized octane measurements have been carried out and consistent data acquired for a large fraction of the hydrocarbons commonly found in commercial gasolines, the same is not true for burning velocities, and consequently the effects of fuel composition on burning velocity are not well understood.

Several approaches have been investigated to boost the burning velocity of a fuel. One approach is to add an additive not normally present in commercial gasoline streams. For example, U.S. Pat. No. 5,354,344 A1 describes a gasoline fuel composition containing 5-50% by volume of the chemical 3-butyn-2-one. This additive is said to improve the flame propagation speed, engine output power, ignitability, and reduce cycle-to-cycle fluctuations, although no assertions are made related to improving vehicle efficiency. However, because this additive is a pure chemical component that requires a multi-step chemical synthesis, its introduction into commercial gasolines at the claimed dosages would involve significant expense, and it is doubtful that the resulting fuel could be made widely available.

U.S. Pat. No. 2,894,830 describes the use of small amounts of boron hydrides in conventional fuels employed for heating or propulsion purposes to increase the combustibility and the velocity of flame propagation of such fuels.

WO 96/40844 A1 and WO 95/33022 A1 describe the introduction of transition metals, alkaline metals, alkaline earths, halogens, group IIIA elements and mixtures thereof into a fuel to increase the fuel's combustion rate. U.S. Pat. No. 4,765,800 discloses that alkali metal salts or alkaline metal earth salts of succinic acid derivatives improve the ignitability of a mixture and shorten flame travelling time. One serious drawback of these approaches is the corresponding emission of uncommon and undesirable pollutants such as boron compounds, metals, or halogens, which could foul engine/exhaust aftertreatment systems and would likely require complex aftertreatment controls to reduce environmental contamination.

An approach to increase the laminar burning velocity of a fuel that forgoes the use of additives is to modify its bulk chemical composition. FIG. 1 shows data from four literature sources that measured laminar burning velocities for a wide range of molecules. The data are from Wagner and Dugger, JACS 77:227 1955, Gibbs and Calcote, J. Chem. Eng. Data, 4:226 1959, Albright, Heath, and Thena, Industrial and Engineering Chemistry 44 10 1952, pp. 2490-1496, and Gerstein, Levine, and Wong, J. Am. Chem. Soc., 73:418 1951. As can be seen from the data, burning velocities are available for only a small number of aromatics. Furthermore, the data are contradictory. For example, Albright et al report ethylbenzene to be the fastest aromatic while Wagner and Dugger report benzene to be the fastest. The paucity of experimental observations and uncertainties in the data render it difficult to elucidate fuel structure effects from these studies. Thus, while there is a general understanding in the art on how fuel structure affects burning velocity for paraffins and olefins, no such understanding exists for aromatics. The present invention resulted from a thorough investigation of the burning velocity for a wide range of aromatic components, from which we have found that the fuel structure effects of aromatics are actually different from that taught in the art. The resulting improved understanding makes the optimization of burning velocity by tuning fuel composition possible.

SUMMARY OF THE INVENTION

The present invention is directed to an unleaded hydrocarbon fuel such as a gasoline boiling range fuel comprising a paraffinic fraction, an olefinic fraction, and an aromatics fraction having an improved laminar burning velocity. The aromatics fraction comprises methyl aromatics and non-methyl alkyl aromatics and the percentage of non-methyl alkyl aromatics in the aromatics fraction is at least 30% on a volume basis. Preferably, the paraffinic fraction is in an amount of 90% or less, the olefinic fraction is in an amount of 30% or less, and the aromatics fraction is in an amount of 70% or less, all calculated on a volume basis. Unless otherwise stated, all percentages listed herein are on a volume basis. The term “paraffinic” as used herein refers to normal, iso, and cycloparaffins, and the term “olefinic” as used herein refers to linear, branched, and cyclo-olefins. The components denoted “non-methyl alkyl aromatics” include molecules such as ethylbenzene, propylbenzene, butylbenzene, and the like, in which a single alkyl chain containing two or more carbons is bonded to the aromatic ring. The components denoted “methyl aromatics” include aromatic molecules such as toluene, o, m, and p-xylenes, trimethylbenzenes, methyl ethylbenzenes, and the like. Components such as oxygenates, di-olefins, benzene, other aromatics and naphthoaromatics may also be included in the hydrocarbon fuel.

The hydrocarbon fuel preferably contains benzene in an amount less than 1% by volume and sulfur less than 30 ppm by weight.

The invention is also directed to an unleaded hydrocarbon fuel comprising a paraffinic fraction, an olefinic fraction, and an aromatics fraction, wherein said methyl aromatics fraction comprises xylenes (dimethyl benzenes) and the percentage of ortho- and para- substituted xylenes is at least 60% on a volume basis.

The invention further relates to a method for making a hydrocarbon fuel such as unleaded gasoline, low sulfur gasoline, and low benzene gasoline having an improved laminar burning velocity. The terms laminar burning velocity and laminar flame speed are often used interchangeably in the literature and this practice will be followed herein.

The method comprises providing a hydrocarbon fuel having a paraffinic fraction, an olefinic fraction, and an aromatic fraction. The aromatic fraction may comprise methyl aromatics and non-methyl alkyl aromatics. The method includes controlling the concentration of the non-methyl alkyl aromatics in the aromatics fraction to at least 30% by volume. Yet another aspect of the invention is directed to controlling the percentage of ortho- and para-xylenes in the xylene fraction to at least 60% by volume. The paraffinic fraction may comprise normal (linear), branched (iso), and cyclo-paraffins, the olefinic fraction may comprise linear, branched, and cyclo-olefins.

The present inventive method and hydrocarbon fuel are advantageous over conventional methods and fuels. Specifically, inventive fuel compositions exhibit increased laminar burning velocities and substantially improved engine thermal efficiencies. A substantially improved thermal efficiency as this term is used in this invention means a relative brake thermal engine efficiency of at least 0.5%, preferably at least 1.5% and most preferably at least 2% greater than the brake thermal efficiency obtained with an unmodified conventional fuel. Likewise, a substantially improved burning velocity as this term is used in this invention means a burning velocity of at least 4%, preferably at least 10% and most preferably at least 15% greater than the burning velocity of an unmodified conventional fuel.

Another advantage of the inventive hydrocarbon fuel composition is that higher burning velocities also improve lean burn engine operation. Lean burn engines are generally known to improve engine efficiency but conventional gasoline blends often burn too slowly to allow a maximum benefit to be extracted. The burning velocity benefits identified in the present invention apply over substantially the entire fuel/air stoichiometry range, i.e., they are not limited to one operating regime such as stoichiometric, lean, or rich operation. As such, they are useful in extending the lean limit of engine operation thereby increasing engine efficiency. Additionally, the faster heat release provided by fast burning fuels maximizes the power and/or torque output of the engine. A significant improvement of torque output enabled by a fuel composition could allow engine downsizing and thus recover additional efficiency benefits from reduced vehicle weight. Additionally, the inventive compositions have the significant advantage that they can be produced from refinery streams and thus have the potential of being supplied in large quantities at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the peak burning velocities of several aromatic hydrocarbon species as reported in Wagner and Dugger, JACS 77:227, 1955, Gibbs and Calcote, J. Chem. Eng. Data, 4:226 1959, Albright, Heath, and Thena, Industrial and Engineering Chemistry, 44:2490 1952, and Gerstein, Levine, and Wong, J. Am. Chem. Soc., 73:418 1951.

FIG. 2 shows a schematic representation of a constant volume combustion vessel used for laminar burning velocity determinations. A) optical arrangement; B) simplified gas diagram.

FIG. 3 shows peak burning velocity data for several aromatic hydrocarbon compounds in the gasoline boiling point range, acquired at T=450K and P=3 atm.

FIG. 4 shows laminar burning velocity data at T=450 K and P=3 atm for several aromatic species as a function of equivalence ratio φ.

FIG. 5 shows laminar burning velocity data at T=450 K and P=3 atm for two fast fuel formulations compared to a reference gasoline according to one embodiment of the present invention.

FIG. 6 shows the relative burning velocities at T=450 K and P=3 atm for a fast and slow fuel blend according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The laminar burning velocities of more than 30 hydrocarbons were measured in a constant volume combustion vessel under temperatures and pressures that approximate in-cylinder conditions. The apparatus is shown schematically in FIG. 2. The measurements were carried out in a stainless steel vessel with a 16.5 cm diameter spherical cavity (volume=2.4 liter) with four windows for optical access. The vessel was housed in a temperature-controlled oven with quartz windows to transmit Schlieren and ignition laser beams. Liquid fuels were pre-vaporized in an 11 liter stainless steel vessel housed outside of the oven. The vaporized fuel mixture was metered into the combustion vessel using a high sensitivity pressure transducer (PT). Measurements were made over a very wide range of equivalence ratios to characterize the burning velocity dependence on stoichiometry. The symbol φ denotes the fuel/air equivalence ratio, wherein a value of φ=1.0 represents a stoichiometric fuel/air mixture. The air charge was admitted next until the desired pressure was achieved. The mixtures were ignited in the center of a spherical vessel with a laser pulse at an initial temperature of 450 K and initial pressure of 3 atm. The data were acquired over a stoichiometry range of a fuel to air ratio (Φ) of from about 0.55 to about 1.30 to determine how the fuel to air ratio affects the burning velocity. Following ignition, the pressure rise in the vessel is monitored with a fast, high dynamic range pressure transducer.

The data of pressure as a function of time data were converted via a thermodynamic analysis to mass fraction burned based on the established approach described by Metghalchi and Keck [Metghalchi, M. and Keck, J. C.; “Burning velocities of mixtures of air with methanol, isooctane, and indolene at high pressure and temperature”, Combustion and Flame, 1982, vol. 48, pp. 191-210]. Data from the pressure-based measurements are extrapolated back to the initial conditions (450 K and 3 atm) to ensure that the fuels are compared under the same temperature and pressure conditions. This method utilizes data in which the flame radius is much greater than the flame thickness, rendering the effects of stretch negligible. The results for ethane and butane acquired under ambient conditions (300 K and 1 atm), for which accurate literature data are available for comparison, were obtained for the purpose of validating the techniques used herein for accurately determining burn velocity.

The results show that, of the fuels studied, methane is the slowest paraffin and ethane the fastest. Generally, olefins have a faster burning velocity than the corresponding paraffins. By corresponding paraffin we mean a paraffin that has the same carbon connectivity as a given olefin, e.g., iso-butene and iso-butane, 2,2,4 trimethyl pentane and 2,4,4-trimethyl-1-pentene, etc. Branched paraffins are slower than non-branched (linear) paraffins, and branched olefins are slower than non-branched (linear) olefins. Aromatics other than benzene are generally slower than the olefins and paraffins, while oxygenates are faster.

The burning velocities of aromatics are illustrated in FIG. 3. As shown, methyl benzenes such as the xylenes and trimethylbenzenes are slower than the non-methyl alkyl aromatics such as ethylbenzene and propylbenzene. Moreover, FIG. 3 shows that among the multi-methyl aromatics such as the xylenes and trimethyl benzenes, in which more than one methyl group is substituted on the aromatic ring, the sites of methyl substitution influence burning velocity, that is, ortho- and para-substituted isomers have a higher burning velocity than the meta-substituted isomers.

One aspect of the present invention relates to a method for blending a fuel such as gasoline to increase laminar burning velocity. Such a blended fuel will yield benefits in any engine (either spark ignition, compression ignition, or a combination thereof) in which flame propagation is operative in consuming the fuel. Generally the method includes controlling the composition of the aromatic component of the fuel as taught herein. We have found that the laminar burning velocity of a fuel increases with the following general changes: a) increasing the concentration of non-methyl alkyl aromatics and decreasing the concentration of methyl aromatics, and b) increasing the concentration of ortho- and para-substituted multi methyl aromatics.

One embodiment of the invention increases the laminar burning velocity of a full-range gasoline by altering the composition in such a way as to increase the concentration of “preferable” compounds and decrease the concentration of “less preferable” compounds, while keeping the overall percentage of olefins, paraffins, and aromatics unchanged. The term “preferable compounds” means compounds that, according to the teaching of this invention, increase the fuel's burning velocity. For example, one embodiment of the invention includes keeping the total concentration of aromatics in the fuel constant while increasing the ratio of non-methyl alkyl aromatics in the aromatic fraction, such as ethylbenzene, n-propylbenzene, iso-propylbenzene, and t-butylbenzene, and/or decreasing the methyl aromatics such as toluene, xylene, and trimethylbenzenes. It has been discovered that the variation in burning velocity between the fastest and slowest aromatics in the gasoline boiling range is about 50%, which is higher than the variations observed among olefins and paraffins in this boiling point range.

Engine and vehicle data obtained indicate that these modifications can translate into a substantial thermal efficiency improvement of at least about 0.5%, preferably at least about 1.5%, and more preferably at least about 2%. For example, according to one embodiment of the invention two fuels with laminar burning velocities that differ by 1% yield a 2% difference in the relative brake thermal efficiency in an engine test.

Thus, according to the present invention, a fuel's burning velocity can be increased by increasing the proportion of non-methyl alkyl aromatics to methyl aromatics, and increasing the proportion of ortho- and para-xylene to m-xylene.

As shown in FIG. 4, the relative ranking of the aromatics persists to both lean and rich conditions, meaning that the improvements in burning velocity achieved by varying the fuel composition may be realized across the entire load-speed operating map of the engine. Stated alternately, since there are no discernible differences between the fuels as a function of fuel/air ratio φ, that is, the relative differences between the fuels are effectively the same under lean, stoichiometric, and rich conditions, there is no basis for defining preferential composition for only a given part of the drive cycle based on flame speed differences.

An embodiment of the present invention relates to a hydrocarbon fuel comprising a paraffinic fraction in an amount of 90% or less, an olefinic fraction in an amount 30% or less, and an aromatics fraction in an amount of 70% or less, wherein said aromatics fraction comprises methyl aromatics and non-methyl alkyl aromatics and the concentration of non-methyl alkyl aromatics in said aromatics fraction is at least 30%. Preferably, the concentration of non-methyl alkyl aromatics in the aromatics fraction may be at least 50%, and more preferably at least 70%.

In another preferred embodiment, the methyl aromatics fraction comprises xylenes and the percentage of ortho- and para-xylene in said xylene fraction is at least 60%. Preferably, the concentration of ortho- and para-xylene in said xylene fraction may be at least 75%, and more preferably at least 90%.

The present invention also relates to a method for making a hydrocarbon mixture in the gasoline boiling point range having an improved laminar burning velocity. The method comprises providing a gasoline comprising a paraffinic fraction, an olefinic fraction, and an aromatic fraction. The paraffinic fraction comprises linear, branched, and cyclo-paraffins, the olefinic fraction comprises linear, branched, and cyclo-olefins, and the aromatic fraction comprises methyl aromatics and non-methyl alkyl aromatics. The method further comprises controlling the concentration of the non-methyl alkyl aromatics in the aromatics fraction to at least 30% and the percentage of ortho- and para-substituted xylene in the xylene fraction to 60%.

A preferred embodiment comprises controlling the percentage of non-methyl alkyl aromatics in the aromatics fraction to at least 50% and the percentage of ortho- and para-substituted xylene in the xylene fraction to 75%.

A most preferred embodiment comprises controlling the percentage of non-methyl alkyl aromatics in the aromatics fraction to at least 70% and the percentage of ortho- and para-substituted xylene in the xylene fraction to 90%. These and other embodiments of the invention will become more apparent to those skilled in this art from the following examples.

It has been found that higher burning velocity correlates with increased efficiency in vehicle tests. Data have been obtained with a prototype vehicle (4-speed ATM, IW=1360 kg) with a 4-cylinder, direct injection gasoline engine. The vehicle was evaluated with a U.S. driving cycle in which lean-burn operation was achieved for half the drive cycle. Multiple test fuels, and a base fuel were evaluated in which the aromatics level, olefin level, and volatility were varied. Laminar burning velocity measurements show that there was about an 11% variation in burning velocity which resulted in about a 2% relative efficiency difference in the vehicle.

EXAMPLE 1

Two model fuels were blended to have a RON and boiling point distribution comparable to a conventional U.S. gasoline. The molecular components were chosen on the basis of maximizing where possible those molecules which have an elevated burning velocity. The fuel composition (all values in weight %) and properties are shown in Table 1.

	TABLE 1
	Fuel FF1	Fuel FF2	REF Gasoline
1-hexene	23.43	3.50
cyclohexane		9.10
methylcyclohexane	5.35	4.99
iso-octane	31.79
1-pentene	5.35	4.16
3-heptene	3.90	16.35
ethylbenzene	30.18	11.26	1.90
toluene		50.64	8.32
c6 isoparaffins			8.79
c7 isoparaffins			6.68
c9 aromatics			6.54
c5 isoparaffins			6.07
c5 olefins			5.37
c11 naphthenes			4.84
n-pentane			4.81
c8 olefins			4.55
n-hexane			4.17
c10 aromatics			4.03
c11 aromatics			3.23
c11 aromatics			3.23
m-xylene			3.13
c8 isoparaffins			2.37
c6 olefins			2.37
c4 olefins			2.11
butane			1.72
c7 olefins			1.50
o-xylene			1.47
n-heptane			1.46
p-xylene			1.13
n-octane			0.63
sum	100.0	100.0	90.41
RON	92.1	92.8	89.8
1The large number of remaining components are present at very small concentrations (<1% each) and are not shown.

The burning velocity data for these fuels and a conventional reference gasoline (REF gasoline) are shown in FIG. 5. It can be seen that the molecular constituents can be preferentially chosen to significantly enhance the burning velocity of the fuel.

EXAMPLE 2

Two fuel blends were prepared containing a single aromatic, olefinic, and paraffinic component. Blend one was composed of iso-octane, 2,4,4-trimethyl-1-pentene, and m-xylene, which are a “slow” paraffin, olefin, and aromatic, respectively. Blend 2 was composed of n-pentane, 1-hexene, and iso-propylbenzene, which are a “fast” paraffin, olefin, and aromatic, respectively. The concentrations of the paraffin, olefin, and aromatic were chosen to approximate those in commercial gasoline. The compositions of these fuels are shown in the table below.

	TABLE 2
			Aromatic
			Methyl/Non-Methyl
	Vol %	Vol %	Alkyl
Component	Fuel 1	Fuel 2	Fuel 1	Fuel 2
n-pentane		60
iso-octane	60		1:0
1-hexene		10
2,4,4-trimethyl-1-pentene	10			0:1
isopropyl benzene		30
m-xylene	30
Total	100	100

The burning velocities of these fuels were determined at 450° K and 3 atm. The results, shown in the FIG. 6, show that a burning velocity increase of 17% was achieved solely by replacing the “slow” paraffins, olefins, and aromatics with “fast” analogues. Thus, preferentially tailoring the molecular structure of paraffins, olefins, and aromatics, without changing the bulk concentration of these constituents, increases burn rate, and by extension, engine efficiency.

Claims

1. An unleaded hydrocarbon fuel comprising a paraffinic fraction, an olefinic fraction, and an aromatics fraction, wherein said aromatics fraction comprises methyl aromatics and non-methyl alkyl aromatics and the percentage of non-methyl alkyl aromatics in said aromatics fraction is at least 30% by volume.

2. The hydrocarbon fuel of claim 1, wherein said paraffinic fraction is in an amount of 90% or less by volume, said olefinic fraction is in an amount of 30% or less by volume, and said aromatics fraction is in an amount of 70% or less by volume.

3. The hydrocarbon fuel of claim 1, wherein the percentage of non-methyl alkyl aromatics in said aromatics fraction is at least 50% by volume.

4. The hydrocarbon fuel of claim 1, wherein the percentage of non-methyl alkyl aromatics in said aromatics fraction is at least 70% by volume.

5. The hydrocarbon fuel of claim 1, wherein said methyl aromatics fraction comprises xylenes and the percentage of ortho- and para-xylene in said xylene fraction is at least 60% by volume.

6. The hydrocarbon fuel of claim 1, wherein said methyl aromatics fraction comprises xylenes and the percentage of ortho- and para-xylene in said xylene fraction is at least 75% by volume.

7. The hydrocarbon fuel of claim 1, wherein said methyl aromatics fraction comprises xylenes and the percentage of ortho- and para-xylene in said xylene fraction is at least 90% by volume.

8. The hydrocarbon fuel of claim 1, further comprising benzene in an amount of 1% or less by volume, and sulfur in an amount of 30 ppm or less by weight.

9. An unleaded hydrocarbon fuel comprising a paraffinic fraction, an olefinic fraction, and an aromatics fraction, wherein said aromatics fraction comprises a xylene fraction and wherein the percentage of ortho- and para-xylene in said xylene fraction is at least 60% by volume.

10. The hydrocarbon fuel of claim 9, wherein the percentage of ortho- and para-xylene in said xylene fraction is at least 75% by volume.

11. The hydrocarbon fuel of claim 9, wherein the percentage of ortho- and para-xylene in said xylene fraction is at least 90% by volume.

12. The hydrocarbon fuel of claim 9, further comprising benzene in an amount 1% or less by volume, and sulfur in an amount of 30 ppm or less by weight.

13. A method for making a hydrocarbon fuel having an improved laminar burning velocity the method comprising:

providing a hydrocarbon fuel comprising a paraffinic fraction, an olefinic fraction, and an aromatic fraction wherein said aromatic fraction comprises methyl aromatics and non-methyl alkyl aromatics; and

controlling the percentage of said non-methyl alkyl aromatics in said aromatics fraction to at least 30% by volume.

14. The method of claim 13, further comprising controlling the percentage said of non-methyl alkyl aromatics in said aromatics fraction to at least 50% by volume.

15. The method of claim 13, further comprising controlling said percentage of said non-methyl alkyl aromatics in said aromatics fraction to at least 70% by volume.

16. A method for making a hydrocarbon fuel having an improved laminar burning velocity the method comprising providing a hydrocarbon fuel comprising a paraffinic fraction, an olefinic fraction, and an aromatic fraction comprising methyl aromatics and non-methyl alkyl aromatics, wherein said methyl aromatics fraction comprises xylenes; and

controlling the percentage of ortho- and para-xylene in the xylene fraction to at least 60% by volume.

17. The method of claim 16, further comprising controlling the percentage of ortho- and para-xylene in the xylene fraction to at least 75% by volume.

18. The method of claim 16, further comprising controlling the percentage of ortho- and para-xylene in the xylene fraction to at least 90% by volume.