Fuels Combustion Research: Supercritical Fuel Pyrolysis

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1. REPORT DATE (DD-MM-YYYYI 04-06-2003

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4. TITLE AND SUBTITLE

Fuels Combustion Research: Supercritical Fuel Pyrolysis

6. AUTHORISl

Mary J. Womat, Elmer B. Ledesma, Philip G. Felton, Joseph A. Sivo, Nathan D. Marsh

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Princeton University Department of Mechanical and Aerospace Engineering Engineermg Quadrangle, Olden Street Princeton, New Jersey 08544-5263

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Supercritical pyrolysis experiments were conducted with toluene, methylcyclohexane, and n-heptane at temperatures up to 585 C, pressures up to 100 atm, and residence times up to 550 sec. Analysis, by HPLC/UV, of the toluene reaction products led to the unequivocal identification of 27 individual PAH, up to 10 rings in size. The experiments showed that PAH yields increased exponentially with pressure. For each of the product PAH, values of the preexponential factor A and the activation volume V* were determined for the pressure-dependent global first-order kinetic rate constant. PAH yields were also extremely sensitive to temperature. At 100 atm and 480 C, PAH of only up to 2 rings were observed; at 535 C, PAH up to 10 rings were formed; at 585 C, solid deposits formed, plugging the reactor. Similar pressure and temperature sensitivities were exhibited by the PAH produced by methylcyclohexane. The nature of the PAH product distributions suggested that condensation reactions, involving the addition of aromatic structures and hydrogen loss, were a dominant mechanism for PAH formation in the supercritical environment.

15. SUBJECT TERMS

supercritical hydrocarbon pyrolysis, polycyclic aromatic hydrocarbons, endothermic fuels, fliel-line deposits, hypersonic aircraft fuels, reaction kinetics, PAH formation, pressure effects

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FUELS COMBUSTION RESEARCH: SUPERCRITICAL FUEL PYROLYSIS

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1 Cover Page

Table of Contents ii

Background and Introduction 1

Experimental Equipment and Techniques 4

Research Results and Discussion 7

Summary of Results 16

Future Work 18

Presentation and Publication of Research Results 18

Acknowledgements 19

References 19

Table 1 22

Figures 23

PIADC Form 63

Background and Introduction

The fuels used in the next generation of hypersonic aircraft will have to operate under very

high pressures and will have to sustain very high heat loads (e.g., 30,000 BTU/min) [1,2]) in

order to meet aircraft cooling requirements. Within the fuel lines and injection system, where

residence times can be many minutes, fuel temperatures and pressures may reach or exceed 540 °C

(813 K) and 150 atm [2]—temperatures and pressures that exceed the critical temperatures and

pressures of most pure hydrocarbons and jet fuels such as JP-7 and JP-8 [3]. At these

temperatures and pressures, the fuel can undergo pyrolytic reactions, which have the potential of

forming solid deposits that can clog fuel lines, foul fuel nozzles, and lead to undesirable or even

disastrous effects for the akcraft.

The current inability to predict solids formation tendencies of fuels under supercritical

conditions has been brought to our attention by Dr. Tim Edwards (Fuels Section, AFRL Turbine

Engine Division, Wright-Patterson Air Force Base, Ohio), who shared with us solid deposition

results for various jet ftiels and n-octane, from scramjet test rigs at United Technologies Research

Center (UTRC) [4]. The tests show that the tendency to produce solid deposits increases in the

order: JP-7 < RP-1 < JP-8+100 < JP-10 < n-octane—an order that would not have been

predicted, based solely on the proportions of paraffins, naphthenes, and aromatics in the fuels.

Clearly we need to know more about the pyrolysis reactions of these fuels under supercritical

conditions. In order to develop reliable fuel systems for high-speed aircraft that will not be subject

to solid deposit formation, we need a thorough understanding of the pyrolysis behavior of

candidate fuels under the supercritical conditions that they will be operating. Of particular interest

are the reactions leading to polycyclic aromatic hydrocarbons (PAH), which serve as precursors to

fiiel-line deposits.

The fact that the fuel pyrolysis environment is a supercritical one introduces several

complexities. With regard to physical properties, supercritical fluids have highly variable

densities, no surface tension, and transport properties (i.e., mass, energy, and momentum

diffusivities) comparable to those of gases. Solvent-solute interactions, absent in the gas phase.

can exhibit huge effects in supercritical fluids, often affecting chemical reaction pathways by

facilitating the formation of certain transition states [5]. Because solvent-solute interactions are

very dependent on pressure, chemical reaction rates in supercritical fluids can be highly pressure-

dependent [5-8]. The kinetic reaction rate constant k has been shown [6,9] to vary exponentially

with pressure, according to the expression

k = Aexp[(-AV*/RT)p]

where A is the preexponential factor (in sec'l) and AV* is the activation volume (in L/mole). As

the equation shows, a negative value of AV* denotes that the reaction is favored by an increase in

pressure. The magnitude of AV* is an index of how sensitive the reaction rate is to pressure. For

gases, AV* is essentially zero; for liquids, AV* is on the order of 10-2 L/mole; for supercritical

fluids, AV* is on the order of 1 to 10 L/mole [9].

For the case of fuel pyrolysis reactions, Stewart et al. [10-12] have demonstrated that reaction

pathways and reaction kinetics indeed differ between the gas phase and the supercritical phase.

Their pyrolysis experiments with decalin and methylcyclohexane in an atmospheric-pressure flow

reactor and in the very supercritical pyrolysis reactor currently in our use show that under

supercritical conditions—^but not in the gas phase at atmospheric pressure—^both decalin and

methylcyclohexane are able to produce methylated Cs-ring intermediates that readily convert to

structures containing 6-membered aromatic rings. These aromatic rings can then serve as kernels

for further cyclic growth to PAH and ultimately solid deposits. For the same two fuels, decalin

and methylcyclohexane, Stewart et al [10-12] also report different global Arrhenius kinetic rate

parameters A and Eg for their supercritical pyrolysis experiments, compared to their gas-phase

experiments. We thus see that reaction pathways and reaction kinetics in the supercritical phase are

substantially different from those in the gas or liquid phase. Therefore even for fiiels whose gas-

phase or liquid-phase pyrolysis behavior is well understood, it is of critical importance to study

their pyrolysis in the supercritical phase, if these fuels are to be considered for fiiture high-speed

aircraft.

To that end, we have begun an experimental research program on the supercritical pyrolysis of

model fiiels. The model fuels include both aromatic and aliphatic components of jet fuels, as well

as methylcyclohexane, an "endothermic" fuel which has attracted interest due to its high heat-

absorbing capacity as it would undergo endothermic dehydrogenation prior to combustion. We

have chosen these well-defined model fuels rather than an actual muWcomponent jet fuel like JP-7

or JP-8, so that we can trace reaction pathways from starting material to PAH to solid deposits—

with the aim of gaining an understanding of the fundamental reaction processes taking place.

Knowledge of individual components' behavior lays the fundamental groundwork for later

examining these components in combination with one another, leading to the eventual

understanding of what takes place with fuels that are more complex mixtures. What we learn of

the pyrolysis behavior of certain components should provide crucial information for the assessment

of solid-deposit formation tendencies in existing fuels and for the formulation of new fuels that

may be more resistant to solid deposit formation.

Our supercritical pyrolysis experiments [14,15] make use of the reactor designed by Davis

[13] and used by Stewart et at [10,12] for earlier supercritical pyrolysis experiments with model

fuels. Since the main focus of our work is the formation of fuel-line deposits during supercritical

pyrolysis and since PAH are precursors to fuel-line deposits, a critical component of our work is

the chemical analysis of large PAH, for which we employ high-pressure Hquid chromatography

(HPLC) with diode-array ultraviolet-visible (UV) absorption detection, a technique ideally suited

for isomer-specific PAH analysis.

In the following, we first describe the experimental equipment and techniques used in the

supercritical pyrolysis experiments. We report the results of our experiments with toluene, n-

heptane, and methylcyclohexane—delineating the regimes of PAH and solid-deposit formation,

discussing the effects of temperature and pressure on PAH product yields, reporting values of

global kinetic rate parameters for individual PAH, and presenting overall PAH-formation reaction

pathways consistent with our experimental observations. We then summarize the key results and

present recommendations for ftiture work.

Experimental Equipment and Techniques

Reactor System

The supercritical fuel pyrolysis experiments have been conducted in an isothermal, isobaric

reactor designed expressly for such experiments by Davis [13] and used by Stewart [10,12] in an

AFOSR-sponsored research program supervised by Professor Irvin Glassman at Princeton

University. Upon his retirement. Professor Glassman made the reactor available to us for

continuing supercritical fuels pyrolysis research.

The reactor system is illustrated in Figure 1. Prior to an experiment, the liquid fuel (99.9+%

pure) is sparged with nitrogen for three hours, as described by Stewart [10], to get rid of any

dissolved oxygen that could introduce auto-oxidative effects [16]. The sparged fuel is then loaded

into a high-pressure nonreciprocating pump, which delivers the fuel to the reactor, as shown in

Figure 1. The reactor itself is a coil of 1-mm i.d., 1.59-mm o.d. capillary tube made of silica-lined

stainless steel. (The silica lining prevents wall-catalyzed deposit formation that occure with unlined

stainless steel [10,13,17].) The reactor coil is immersed in a temperature-controlled fluidized-

alumina bath, which ensures isothermality throughout the reactor length. As indicated in Figure 1,

the entrance and exit lines of the reactor are passed through a water-cooled heat exchanger to

ensure a controlled thermal history and residence time. Exiting the heat exchanger, the quenched

reaction products pass through a stainless steel filter (hole size, 5 ^m) and on to a six-position

high-pressure valve, for product collection, A dome-loaded back-pressure regulator, downstream

of the valve, controls the system pressure, to within +0.2 atm, up to a maximum of 110 atm. A

burst disk, located upstream of the reactor, provides a safe flow outlet, in case of over-

pressurization.

Reactor residence time is varied by changing the length of the reactor coil. The reactor system

is capable of operating at temperatures up to 860 K, pressures up to 110 atm, and residence times

up to 3600 sec—operating ranges relevant to those envisioned for fuel systems in future

hypersonic aircraft [2]. As documented by Davis [13] and Stewart [10], the reactor has been

designed to meet Cutler's [18] and Lee's [19] criteria for idealization as plug flow, with regard to

species concentration profiles. The resulting radially uniform species concentrations, coupled with

the reactor's constant-temperature and constant-pressure operation, render this reactor ideal for

supercritical pyrolysis kinetics experiments.

Pyrolysis experiments have been performed with the following model fuels: n-heptane

(critical temperature, 267 °C', critical pressure, 27 atm), an aliphatic component of jet fuels;

methylcyclohexane (critical temperature, 299 °C; critical pressure, 34 atm), a model fuel of interest

to the Air Force due to its high heat-absorbing capacity as it would undergo an endothermic

dehydrogenation reaction in the fuel lines, prior to combustion; and toluene (critical temperature,

319 °C; critical pressure, 41 atm), an aromatic component of jet fuels as well as the

dehydrogenation product of the endothermic fuel methylcyclohexane.

Product Analysis

At the conclusion of a pyrolysis experiment, the liquid-phase reaction products are removed

from the high-pressure collection valve and transferred to a vial. Two separate 10-pL aliquots of

the product solution are removed for injection onto the gas chromatograph, which analyzes the

smaller aromatic products (1 to 4 rings). The remainder of the product solution (~1 mL) is

prepared for high-pressure liquid chromatography (HPLC), which analyzes the larger aromatic

products (> 3 rings).

Gas chromatographic analysis of the supercritical fuel pyrolysis products is performed on an

Agilent Model 6890 gas chromatograph (GC) with flame-ionization detector (FID), in conjunction

with an Agilent Model 5973 mass spectrometer (MS). A sample volume of 10 p.L is injected by

syringe, through a split injector, onto an HP-5 fused silica capillary column of length, 30 m;

diameter, 0.25 mm; and film thickness, 0.1 ixm. The column temperature is programmed to hold at

40 °C for the first 3 minutes; it is then ramped at 4 °C /min to 300 °C, where it is held for 15

minutes. The GC/FID/MS instrument is used to quantify 1- to 4-ring aromatic products—all of

which are identified by matching retention times and mass spectra with those of reference

standards.

The portion of the product solution reserved for HPLC analysis is concentrated in a Kudema-

Danish apparatus and exchanged, under nitrogen, into 100 jxL of dimethylsulfoxide, a solvent

compatible with the solvents used in the HPLC method employed for PAH analysis. During the

concentration and solvent-exchange procedure, portions of the more volatile aromatics such as the

1- and 2-ring species, are lost to vaporization; hence these Hghter aromatic products are quantified

by gas chromatographic analysis, as described above.

For analysis of the large aromatic products (> 3 rings) by HPLC, a 20-\iL aliquot of the

product/dimethylsulfoxide solution is injected onto a Hewlett-Packard Model 1050 high-pressure

liquid chromatograph, coupled to a diode-array ultraviolet-visible (UV) absorbance detector. The

chromatographic separation method [20,21], optimized for large PAH analysis, utilizes a reversed-

phase Vydac 201-TP octadecylsilica column of particle size, 5 \im; inner diameter, 4.6 mm; and

length, 250 mm. A time-programmed sequence of solvents—acetonitrile/water, acetonitrile, and

dichloromethane—is pumped through the column, and the PAH product components elute in the

order of increasing molecular size. UV absorbance spectra are taken, every 0.6 seconds, of the

exiting components, which are then identified by matching the UV absorbance spectra (and elution

times) with those of commercially available and specially synthesized reference standards.

Quantification of each species is based on extensive calibration with reference standards, over a

broad range of concentrations,

HPLC/UV is particularly well suited for analyzing the large PAH molecules that are

precursors to fuel-line solid deposits for two reasons: 1) Large PAH are too involatile to be

analyzed by gas chromatography but are amenable to analysis by HPLC. 2) For large PAH there

are many different isomeric structures possible for a given CxHy formula. (For example, there are

65 possible isomers of 6-ring PAH of formula C24H14 [22].) The product identification method,

therefore, must be able to unequivocally distinguish between different PAH isomere. HPLC/UV

permits this unequivocal identification of PAH, since the identification is made by matching each

product component's unique UV absorbance spectrum with that of a reference standard. Through

the years, the principal investigator has amassed a library of hundreds of UV spectra of PAH,

many of which have been obtained from specially synthesized reference standards, expressly for

this purpose. In previous pyrolysis and combustion studies, the principal investigator has

successfully employed HPLC/UV to analyze complex mixtures of product PAH from ftiels such as

coal [23-26], anthracene [20,21,27], benzene [28], and catechol [29]. It should be emphasized

that the HPLC/UV means of analyzing large PAH is a very highly specialized technique—a

capabiMty of only a few laboratories in the world.

Research Results and Discussion

Pyrolysis of Toluene: PAH Products Identified

Pyrolysis experiments have been conducted with toluene at temperatures up to 585 ^C,

pressures up to 100 atm, and residence times up to 550 sec. Over the ranges of conditions

investigated, the highest-yield aromatic products from our supercritical toluene pyrolysis

experiments are benzene, the three xylenes, and the ten bi-toluyls. Figure 2 presents the yields of

these major products, measured by GC/FID/MS, as functions of pressure, for experiments run at

535 °C and 550 sec. All of these products are observed in gas-phase toluene pyrolysis experiments

[30] and result from straight-forward reaction steps. Displacement of toluene's methyl group by H

gives benzene. Displacement of any of toluene's aryl hydrogens by methyl gives the xylenes. The

various bi-toluyls arise from abstraction of H from toluene (at any of various sites) to form a

radical, which then displaces an H (at any of various sites) on another toluene molecule.

Activation energies [30] for the various types of bond-breaking steps in these reactions are shown

in Figure 3. It is important to note that to form any of the reaction products in Figure 2 requires

breaking of only single bonds, either C-H or C-C, and does not involve the breaking of the

aromatic ring in toluene's structure.

Figure 4 presents an HPLC chromatogram of the products of supercritical toluene pyrolysis at

535 °C, 550 sec, and 100 atm, the highest pressure in Figure 2. Depicted in Figure 4 are the

structures of the twenty-seven 2- to lO-ring PAH that have been unequivocally identified by

HPLC/UV in this product mixture. (The cluster of large peaks in the 12- to 20-minute retention

time range corresponds to the bi-toluyls, which are better resolved by GC and thus not individually

labelled with product structures in Figure 4. PAH such as phenanthrene and anthracene, which

elute on the HPLC in the same time range as the bi-toluyls, are quantified by GC instead of HPLC,

to avoid signal interference.) Most of the 2- to 5-ring PAH in Figure 4 have been identified by

others [30,31] as products of toluene pyrolysis at subcritical pressures. Those studies, however,

do not report PAH of > 5 rings, such as the nine 6- to lO-ring PAH found in the supercritical

pyrolysis products of Figure 4.

Of particular note in Figure 4 are the four very large PAH of 8, 9, and 10 fused aromatic

rings—^benzo[a]coronene, benzo[pgr]naphtho[8,1,2-ftc<f|perylene, naphtho[8,1,2-flfec]coronene,

and ovalene—none of which have ever before been identified as products of toluene pyrolysis or

as products of supercritical pyrolysis of any fuel. The UV spectral matches confirming the

identifications of these 8- to 10-ring PAH, as well as the 6- and 7-ring PAH, are displayed in

Figures 5-8. It should be noted that it is rare for any of the 8- to 10-ring PAH to be observed in

fuel products—^partly because few researchers have the analytical techniques or the specially

synthesized reference standards [32] necessary to identify these species and partly because

formation of these large PAH requires extreme conditions. For example, one of the only other

cês in which the large PAH ovalene has been shown [33] to be produced is from subcritical fuel-

rich combustion of naphthalene/ethylene mixtures at 1375 "^C (840 ^C hotter than the temperature in

Figure 4). The fact that the monocyclic fuel toluene can produce the 10-ring ovalene at a

temperature of only 535 °C points to the huge effect of the supercritical pressure conditions in

facilitating the formation of large PAH, which can be precursors to solid deposits.

Temperature Effects and Solid Deposits Formation

That these large PAH are precursors to solid deposits is corroborated by the fact that our

supercritical toluene pyrolysis experiments at 100 atm and 585 °C (just 50 °C higher than the

temperature corresponding to Figure 4) result in repeated plugging of the reactor, due to solid

deposit formation. This extreme sensitivity to temperature, in the high-pressure supercritical

pyrolysis environment, is further illustrated by comparison of Figure 4, the chromatogram of

toluene products at 100 atm and 535 °C, with Figure 9, the chromatogram of toluene products at

8

100 atm and 480 °C. Figure 9 shows that at 480 °C, just 55 °C lower than the temperature in

Figure 4, no PAH of greater than 2 aromatic rings are detected. In addition, the bi-toluyls formed

at 480 °C are a factor of 20 smaller in yield than those formed at 535 °C. We thus see that at

supercritical pressures, the formation of bi-toluyls, PAH, and solid deposits are all extremely

sensitive to temperature: At 100 atm, within a hundred-°C span of temperature, there is transition

from a regime in which only bi-toluyls and small PAH are produced, to one in which large PAH

are produced (and smaller ones in more abundance), and then to a regime in which solid deposits

are produced.

Pressure Effects

The results from the supercritical toluene pyrolysis experiments show that in addition to being

very sensitive to temperature, PAH formation in a supercritical pyrolysis environment is also very

sensitive to pressure. Figures 10-23 present the yields, as functions of pressure, of the 2- to 10-

ring PAH produced by toluene pyrolysis at 535 ''C. As demonstrated in these figures, yields of all

the PAH increase continuously with pressure, rising particularly dramatically at pressures above

the toluene critical pressure of 41 atm.

Figure 15 reveals a result peculiar to high-pressure toluene pyrolysis. Unlike results observed

for toluene at atmospheric pressure [31], Figure 15 reveals that in the supercritical toluene

experiments, yields of anthracene exceed those of its C14H10 isomer phenanthrene. A similar

finding is reported [30] for toluene pyrolysis at 10 atm. Both of these C14H10 PAH, anthracene

and phenanthrene, can result from the combination of toluene and benzyl radical, as illustrated in

Figure 24 (adopted and extended from Colket and Seery [30]). The observed dominance of

anthracene over phenanthrene suggests that at high pressures, union of the benzyl radical to an aryl

site of toluene (top pathway of Figure 24) is preferred over union at the benzyllic site (bottom

pathway). This observation on the relative yields of the C14H10 PAH illustrates the importance of

pressure in determining reaction pathway. The dominance of pyrene over its CieHio isomer

fluoranthene, in Figure 16, and the dominance of benzo[f î]perylene over its C22H12 isomer

indeno[l,2,3-crf]pyrene, in Figure 20, are also results peculiar to this high-pressure environment

and contrary to observations in atmospheric-pressure pyrolysis systems [20,26,28,29]. These

differences in relative abundances within isomer families suggest that the mechanisms for PAH

formation in the high-pressure environment differ from those in the atmospheric-pressure

environments.

PAH Formation Mechanisms

Figure 24 shows how anthracene and phenanthrene can be formed from supercritical toluene

pyrolysis, but the question arises as to how others of the toluene product PAH in Figure 4 can be

formed. Most of the literature on PAH formation from fuels combustion and pyrolysis invokes

some version of the Ca-addition mechanism [34,35], in which sequential addition of 2-carbon

hydrocarbons such as ethylene, acetylene, or their radicals leads to aromatic structures of

successively larger ring number. This mechanism, widely applicable in high-temperature,

subcritical pyrolysis environments, does not, however, apply to our supercritical toluene pyrolysis

environment in which temperature is much lower, acetylene is not formed, toluene conversion is

very low, and product selectivity is high {i.e., for a given number of rings, only certain species are

found, out of large numbers of possibilities,). The absence, from Figures 4 and 9, of C2-addition

products—such as the cyclopenta-fused PAH and ethynyl-substituted PAH in Figure 25, which

we routinely see in high-temperature, atmospheric-pressure pyrolysis of a variety of fuels

[23,27,29,36-39]—confirms that the C2-addition mechanism is not active under the conditions of

our supercritical toluene pyrolysis experiments.

In the lower-temperature, high-pressure reaction environment of our supercritical pyrolysis

experiments, in which there is no evidence of aromatic ring rapture, it would be logical that the

larger PAH would be produced from combination of the aromatic building blocks available.

Figure 24 illustrates how two of the product PAH in Figure 4 can be formed by combination of

toluene and its most readily formed radical, benzyl. Although toluene is the most plentiftil species

in our reaction environment, other aromatic building blocks are available: the major products from

Figure 2—^benzene, the xylenes, and the bi-toluyls—as well as the product PAH themselves.

Figure 26 shows how three of the products of Figure 4—^pyrene, coronene, and ovalene—can be

10

formed by combinations of the xylenes and their radicals. Subsequent methyl displacement of an

aryl hydrogen on either pyrene or coronene would lead to the 1-methylpyrene or 1-methylcoronene

in Figure 4,

Figures 27-29 illustrate postulated schemes for the formation of other large PAH observed in

Figure 4: the 8-ring benzo[p^r]naphtho[8,l,2-£>cd]perylene (Figure 27), the 6-ring

benzo[gî]perylene and the 8-ring benzo[a]coronene (Figure 28), and the 9-ring naphtho[8,l,2-

afcc]coronene (Figure 29)—all from combining toluene, bi-toluyls, xylenes, benzene, and/or their

radicals. Mechanisms of the types in Figures 24 and 26-29—in which the building blocks are

particular aromatic units and not small species such as C2—are more likely to lead to product

distributions of the kind in Figure 4, in which a high degree of product selectivity is evident (i.e.,

only certain species of a given ring number are produced and not others).

Global Kinetics for PAH Formation and Parameters for the Pressure-Dependent Rate Constant

The pressure-dependent product yield data of the type in Figures 10-23 permit us to determine

the pressure dependency of the global kinetic rate constants for formation of PAH in supercritical

toluene pyrolysis. For our global kinetic analysis, we assume that the rate of production of each

product B is first order in the concentration of toluene A, so

d[B]/dt = k[A] (1)

where k is the global kinetic rate constant (in sec-1) for the formation of B. For the range of

conditions examined in our supercritical toluene pyrolysis experiments, the level of toluene

conversion is very small, less than 1 %, so the concentration of A is effectively constant with tune,

at its initial value of [A]o. Therefore, integration of (1) over time gives:

[B] = k[A]ot (2)

or [B]/[A]o = kt (3)

where [B]/[A]o is just the yield of B, as plotted in Figures 10-23.

For constant temperature, the pressure dependency of the rate constant k is given [9] as:

k = A exp[(-AV*/RT)p] (4)

11

where A is the preexponential factor (in sec'l) and AV* is the activation volume (in L/mole),

defined as the difference between the partial molar volume of the transition state and the partial

molar volimae of the reactants. Substitution of (4) into (3) gives:

[B]/[A]o = At{exp[(-AV*/RT)p]} (5)

Therefore a set of product yield data at constant temperature and residence time but varying

pressure lends itself to determining AV* and A, if the data conform to the assumed first-order

global kinetics.

Using the data from our supercritical toluene pyrolysis at 535 "C and 550 sec, we fit the

experimentally measured yield/pressure data for each product to Equation (5), determining the

values of AV* and A that best fit the data for each product species. A yield/pressure curve is then

generated from the derived values of AV* and A, to examine how well the experimental data

conform to the assumed firet-order behavior.

Figures 30-34 and Table 1 show the results of this exercise for the PAH product yield data

from supercritical toluene pyrolysis at 535 °C and 550 sec. In Figures 30-34, each of the filled

circles is an experimentally measured product yield. Each curve is the one generated from the

values of AV* and A that best fit the data in the form of Equation (5). The close matching of the

data points and curves for each of the PAH in Figures 30-34 shows that the data conform veiy well

to the assumed first-order global kinetics model. The high values of the correlation coefficient R2

in Table 1 show that all of the product PAH—from 1-ring to 9-ring aromatics—conform well to

this type of first-order global kinetic treatment.

Table 1 also reports the values of In A and AV* that best fit the experimental data for each

product. The negative values of AV* of couree indicate that higher pressures favor formation of

the PAH products, as the experiments demonstrate. The magnitudes of the AV* values in Table 1

are typical for reactions in supercritical fluids (two orders of magnitude higher than for liquid-

phase reactions) [6,9]. For an activation volume of -3L/mole at 535 °C, it takes only a 15.3-atm

rise in pressure to double the formation rate of the product PAH; for an activation volume of -4

L/mole, the formation rate doubles with a pressure increase of only 11.5 atm. The activation

12

volumes in Table 1 are a good indicator of just how highly pressure-sensitive PAH formation is in

the supercritical toluene pyrolysis environment. With respect to solid deposit formation, it may be

significant to note from Table 1 that the largest PAH are the ones whose formation rates are the

most sensitive to pressure.

Pyrolysis ofn-Heptane and Methylcyclohexane

The supercritical pyrolysis reactor of Figure 1 has also been employed in experunente with re-

heptane, a model fuel representative of aHphatic components of jet fuels, and methylcyclohexane, a

model endothermic fuel. Under the conditions investigated—^temperatures up to 570 °C, pressures

up to 100 atm, and residence times up to 30 sec—^pyrolysis of «-heptane has not produced

detectable quantities of PAH or solid deposits. Longer residence times should be examined,

though, before any conclusions are drawn about n-heptane—especially since the closely related

alkane n-octane has proven to be prone to producing solid deposits in UTRC's scramjet test rigs

[4].

For the case of methylcyclohexane, subjected to the same ranges of experimental conditions,

analysis of the reaction products reveals that for the 30-sec residence time employed and the entke

range of pressures investigated, PAH are not produced at detectable levels at temperatures < 430

°C. At the highest temperatures of 530 and 570 °C, PAH are produced at all pressures. At the

intermediate temperature of 470 °C, PAH are detected at all pressures but in only minimally

detectable amounts at the lowest pressure of 20 atm.

As in the case of the supercritical toluene experiments, the supercritical pyrolysis experiments

with methylcyclohexane show that an increê in pyrolysis temperature brings about an increase in

PAH production as well as an increase in the molecular sizes of the PAH formed. Figure 35

depicts an HPLC chromatogram of the products from methylcyclohexane pyrolysis at 570 °C and

100 atm, the highest temperature and pressure examined. Even though Figure 35 contains some

unresolved components (due to the high degree of alkylation), twenty-three PAH have been

unequivocally identified among these methylcyclohexane pyrolysis products—all but two (2,3-

dimethylnaphthalene and benzo[fl]fluorene) of which are also observed as supercritical toluene

13

pyrolysis products in Figure 4. The UV spectral matches confirming the identities of two of the

methylcyclohexane products of Figure 35, benzo[a]pyrene and anthanthrene, are displayed in

Figure 36.

As illustrated in Figure 35, supercritical pyrolysis of methylcyclohexane at 570 °C and 100

atm produces a variety of PAH, ranging in size from one to seven aromatic rings, and including

several classes of compounds: 10 benzenoid PAH (those with only six-membered rings), 3

fluoranthene benzologues (those with an internal five-membered ring), 3 indene benzologues

(those with a five-membered ring containing a methylene carbon), and 6 methylated PAH. The

high abundance of methylated PAH distinguishes these methylcyclohexane producte from products

of purely aromatic fuels [20] and undoubtedly results from the abundance of methyl fragments

readily produced from pyrolysis of a hydrogen-rich endothermic fuel such as methylcyclohexane.

Additional methylated PAH are thought to be among the products not yet identified in Figure 35

due to the coelution of species and the consequent superposition of peaks in the product

components' UV spectra.

Similar again to the results of the toluene experiments, the supercritical pyrolysis experiments

with methylcyclohexane show that pressure is a huge factor in determining PAH yields. Figures

37 and 38 illustrate the effect of pressure on the yields of 2-, 3-, and 4-ring PAH produced from

methylcyclohexane pyrolysis at 530 °C and 30 sec. As these figures indicate, all PAH increase in

yield as pressure is increased—^the greatest jump occurring in the interval from 80 to 100 atm, just

as in the case of toluene.

For methylcyclohexane, the increase of PAH production with increasing pressure can be tied

to the findings of Stewart [10]. As proposed by Stewart [10] and demonstrated in Figure 39,

methylcyclohexane pyrolysis produces an intermediate, the methylhexenyl radical, which either in

the gas phase at atmospheric pressure or in the supercritical phase produces p-scission products

such as ethene and propene. In the very dense environments characteristic of supercritical

pressures, however, diffusion of p-scission products (away fi-om one another) is impaired and

collision processes are favored, enhancing carbon-carbon bond formation and facilitating the

14

production of cyclic stractures. Under supercritical conditions, therefore, the intermediate

methylhexenyl radical produces dimethylcyclopentane [10], a hydrocarbon containmg a methylated

five-membered ring. Such five-membered-ring species have been shown [10, 40] to undergo

facile conversion to structures containing six-membered aromatic rings. It thus makes sense that

the higher pressures that enhance production of five-membered-ring species also enhance

production of PAH, as demonstrated in the methylcyclohexane experiments (Figures 37 and 38).

Products of Supercritical Pyrolysis ofJP-7

The above results from our laboratory-scale supercritical pyrolysis reactor were discussed, in

part, with Dr. Tim Edwards and his colleagues at Wright-Patterson Air Force Research

Laboratory, during the Principal Investigator's visit there in February, 2002. Subsequent

exchanges with Dr. Edwards and his sharing of scramjet test results on solid deposition prompted

us to perform HPLC analyses on samples from the scramjet tests.

Figure 40 shows the results of our HPLC/UV analysis of producte from JP-7 fuel subjected to

supercritical conditions in a scramjet test rig [41] at UTRC in East Hartford, Connecticut. Due to

the huge number of constitoents in JP-7 fuel, the product mixture is of course extremely complex,

as evidenced by the "hump" of not fully resolved components in the chromatogram of Figure 40.

Despite this complexity, we are able to unequivocally identify, in this product sample, the 22 PAH

whose structures appear in Figure 40. Except for 1-methylanthracene (whose signal would be

obscured by bi-toluyls in Figure 4), all of the PAH in the supercritical JP-7 products of Figure 40

are also products of our supercritical toluene pyrolysis experiments; most are also in the

methylcyclohexane PAH products of Figure 35. Relevance is thus established between our

laboratory-scale supercritical pyrolysis experiments with the model fuels and the larger-scale

scramjet tests with actual jet fuels. Dr. He Huang, Fuels Lab Leader at UTRC, who supplied the

JP-7 product sample to us, is planning to visit our laboratory so that we might discuss our results

and explore opportunities for collaboration.

15

Summary of Results

PAH from toluene pyrolysis. Using a reactor specially designed for supercritical fuels

pyrolysis studies, we have conducted supercritical pyrolysis experiments with the model fiiel

toluene at temperatures up to 585 °C, pressures up to 100 atm, and residence times up to 550 sec.

Analyses of the reaction products by GC/FID/MS and by HPLC/UV, an isomer-specific technique

ideally suited for the analysis of PAH (precursors to solid deposits), show that the major aromatic

products of supercritical toluene pyrolysis are benzene, the xylenes, and the bi-toluyls—all of

which can form without the breaking of toluene's aromatic ring. HPLC/UV analysis has

unequivocally identified 27 individual PAH of up to 10 rings; for many of the 27, it is the first time

that they have ever been identified as toluene products or as the products of supercritical pyrolysis

of any fuel.

Temperature sensitivity and solids formation. The supercritical toluene pyrolysis experiments

show that PAH production is extremely sensitive to temperature, especially at high pressures.

Results at 100 atm and 550 sec show that at 480 °C, only bi-toluyls and 1- and 2-ring aromatics are

formed; at 535 °C, yields of the bi-toluyls increase by a factor of 20, and PAH up to 10 rings are

formed; at 585 °C, solid deposits form, plugging the reactor. Evidence is thus established that

PAH are indeed precursors to the solid deposits.

PAH product distribution. The nature of the PAH product distribution from the supercritical

conditions is hugely different from those of atmospheric-pressure gas-phase environments.

Relative proportions of PAH within a given isomer family ( e.g., C14H10, CieHio, and C22H12)

are markedly different. The high pressures of the supercritical environment permit the formation,

at relatively low pyrolysis temperatures, of large-ring-number PAH, which at atmospheric pressure

would form only at temperatures many hundreds of degrees higher.

PAH formation mechanisms. The PAH product distribution from supercritical pyrolysis of

toluene shows no evidence of the aromatic ring in the toluene structure ever having broken and no

evidence of the C2-addition mechanism commonly invoked for PAH growth in subcritical

pyrolysis and combustion environments {Le., C2-addition products such as cyclopenta-fused PAH

16

and ethynyl-PAH are not present). Condensation reactions, involving the addition of aromatic

structures (e.g., toluene, benzene, xylenes, toluene dimers, and their radicals) present in the

reaction environment, followed by dehydrogenation, appear to be responsible for PAH formation

in the supercritical toluene pyrolysis environment.

Pressure sensitivity and global kinetics. PAH yields are extremely sensitive to pressure,

increasing exponentially as pressure exceeds the critical pressure. For each of the PAH at 535 °C,

the experimentally measured yield/pressure data conform well to a first-order global kinetics

model—^permitting determination, for each product PAH, of the preexponential factor A and the

activation volume AV* used in the pressure-dependent expression for the kinetic rate constant in

supercritical reactions: k = A exp[(-AV*/RT)p]. For most of the PAH, derived values of AV* lie

between -3 L/mole and -4 L/mole, signifying the doubling of PAH formation rates by pressure

increases of only 15.3 or 11.5 atm, respectively. With respect to solid deposit formation, it is

significant to note that the largest PAH are the ones whose formation rates are the most sensitive to

pressure.

Pyrolysis of other model fuels. Under the ranges of experimental conditions examined, n-

heptane does not produce measurable amounts of PAH or solid deposits, but longer residence

times should still be examined. For methylcyclohexane, PAH are not produced at detectable levels

at temperatures < 430 ^C; at temperatures > 470 oC, PAH are produced at all pressures. The

twenty-two individual PAH identified in the methylcyclohexane reaction products are also found in

the toluene pyrolysis products. PAH yields fi-om methylcyclohexane pyrolysis also exhibit the

extreme sensitivities to temperature and pressure demonstrated by those from the toluene

experiments.

Relevance to jet fuels. HPLC/UV analyses of the supercritical pyrolysis products of JP-7,

supplied by Dr. He Huang of UTRC, show that most or all of the PAH identified as products from

JP-7 are also among the PAH produced in our supercritical toluene pyrolysis experiments.

Relevance is thus estaWished between our laboratory-scale supercritical pyrolysis experiments with

the model fiiels and the larger-scale scramjet tests with actual jet fuels.

17

Future Work

The above findings from our supercritical fuels pyrolysis research suggest the need for

additional experiments along the following lines:

(1) Assuming global first-order kinetics for PAH formation, we have determined values of the kinetic parameters A and AV* for toluene pyrolysis at 535 ''C and 550 sec. Toluene pyrolysis experiments at additional temperatures need to be conducted in order to determine how A and AV* vary with temperature.

(2) The toluene results to date show that at 550 sec, 100 atm, and 535 °C, toluene conversion is low and yields of large PAH are on the order of only 10 (xg/g. At a temperature just 50 °C higher, though, solid deposits are produced in sufficient amount to plug the reactor. In order to better understand how the transition occurs from small amounts of large PAH to large amounts of solid deposits, additional toluene pyrolysis experiments need to be performed at small increments of pressure (~5 atm) and at small increments of temperature (-10 °C) within the high-temperature (535-585 °C), high-pressure (60-100 atm) ranges of this "near incipient" zone for solids formation.

(3) Results have been obtained on PAH and solid deposit formation from toluene, an aromatic component of jet fuels. Jet fuels contain aliphatic as well as aromatic components, however. In order to examine how PAH formation from an aromatic component of jet fuel may be affected by the presence of aliphatic components, pyrolysis experiments should be conducted with toluene doped with varying amounts of n-heptane.

(4) One of the important findings of the supercritical toluene experiments is that PAH formation in that environment does not require breaking of the aromatic ring in toluene. Instead the aromatic ring remains in tact to serve as a building unit for the PAH, and presumably eventually die solid deposits. If the same result holds for other aromatic components of jet fuels, then larger-ring-number aromatic components, though present in smaller amounts, could play significant roles in the formation of large PAH and solids. In order to examine this question, pyrolysis experiments should be conducted with 1-methylnaphthalene, a 2-ring PAH identified in jet fiiels [42], to see (a) if even larger PAH than ovalene (the largest one identified in the toluene products) are formed, (b) if less severe conditions (lower T, p, t) are needed to produce high-ring-number PAH; and (c) if solid deposits are formed more readily than in the case of toluene.

(5) Pyrolysis experiments with «-heptane and with methylcyclohexane need to be conducted at longer residence times in order to explore conditions more likely to foster solid deposit formation.

Presentation and Publication of Research Results

Our results from the supercritical pyrolysis research are the subjects of two papers that are

currently in preparation for submission to journals. In addition to the AFOSR Contractors'

Review meetings, the results have been or will be presented at the following conferences:

18

Ledesma, E. B., Sivo, J. A., and Womat, M. J., "Polycyclic Aromatic Hydrocarbons Produced from the Supercritical Pyrolysis of Methylcyclohexane," Two-Hundred Twenty-Firet National Meeting of the American Chemical Society, San Diego, California, April, 2001.

Ledesma, E. B., Felton, P. G., Sivo, J. A., and Womat, M. J., "Pyrolysis of Toluene under Supercritical Conditions," Twenty-Ninth International Symposium on Combustion, Sapporo, Japan, July, 2002.

Womat, M, J., Ledesma, E. B., Felton, P. G., and Sivo, J. A., "Polycyclic Aromatic Hydrocarbons from the Supercritical Pyrolysis of Toluene," Two-Hundred Twenty-Fifth National Meeting of the American Chemical Society, New Orleans, Louisiana, March, 2003.

Womat, M. J., Ledesma, E. B., Felton, P. G., and Sivo, J. A., "Pressure-Dependent Global Kinetics Rate Parameters for the Formation of PAH from Supercritical Toluene Pyrolysis," Eighth International Congress on Combustion By-Products," Umea, Sweden, June, 2003.

Ledesma, E. B., Felton, P. G., Sivo, J. A., and Wornat, M. J., "Formation of Polycyclic Aromatic Hydrocarbons from the Supercritical Pyrolysis of Toluene," Nineteenth International Symposium on Polycyclic Aromatic Compounds," Amsterdam, the Netherlands, September, 2003.

Acknowledgements

We gratefully acknowledge the AFOSR, Grant Number F49620-00-1-0298, for support of

this research. We thank Dr. Elmer Ledesma, Dr. Philip Felton, Mr. Nathan Marsh, and Mr.

Joseph Sivo for technical contributions to this work. We also thank the following for reference

standards and/or UV specfra of PAH: Dr. Arthur Lafleur and Ms. Elaine Plummer, of the

Massachusetts Institute of Technology; Professor Maximilian Zander, of Rtltgerswerke; and Dr.

John Fetzer, of Chevron Research.

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4. Edwards, T., personal communication, AFRL, Wright-Patterson, Ohio, June 12, 2002.

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21

Table 1. Kinetic parameters, the pre-exponential factor A and the activation volume AV*, determined for individual PAH produced by toluene pyrolysis at 535 °C, 550 sec, and pressures from 20 to 100 atm. The parameters are used in the pressure-dependent expression for the kinetic rate constant, Equation (4) in the text: k=Jexp[(-AV*/RT)p]. R^ is the correlation coefficient for how well the experimentally measured yield data match the assumed first-order global kinetics. Equation (5) in the text.

Name In A is') AV* (L mol') R^

1- and 2-rmg aromatics benzene -18.2 -2.95 0.993 indene -21.8 -2.93 0.963 naphthalene -20.0 -2.54 0.980 1 -methylnaphthalene -23.6 -3.49 0.972 2-methylnaphthalene -22.1 -3.83 0.990 diphenylmethane -19.3 -2.20 0.983

3-ring PAH flourene -25.3 -4.03 0.988 phenanthrene -23.1 -3.15 0.993 anthracene -21.4 -2.61 0.989

4-ring PAH flouranthene -26.1 -3.75 0.997 pyrene -23.3 -3.64 0.997 benz[a] anthracene -24.7 -2.96 0.992 chrysene -25.3 -2.88 0.982

5- and 6-ring PAH benzo[a]pyrene -25.6 -3.66 0.998 indeno[l ,2,3-cd]pyrene -25.5 -3.06 0.989 benzo [gAi]perylene -24.5 -3.74 0.998

7-, 8-, and 9-ring PAH coronene -23.6 -3.41 0.994 1 -metliylcoronene -23.8 -3.05 0.969 benzo[a]coronene -30.2 -5.64 0.999 naphtho[8,1,2-fl6c]coronene -30.2 -6.56 0.999

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Principal Investigator Annual Data Collection (PIADC) Survey Form

NOTE: If there is insufficient space on this survey to meet your data submissions, please submit additional data in the same format as identified below.

PI DATA

Name (Last, First, MI): Womat, Mary J.

Institution Princeton University_

Contract/Grant No. F49620-00-1-0298

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Faculty 1 Post Doctorates 1 Graduate Students 1 Other 2_

PUBLICATIONS RELATED TO AFOREMENTIONED CONTRACT/GRANT

NOTE: List names in the following format: Last Name, First Name, MI

Include: Articles in peer reviewed publications, journals, book chapters, and editorships of books.

Do Not Include: Unreviewed proceedings and reports, abstracts, "Scientific American" type articles, or articles that are not primary reports of new data, and articles submitted or accepted for publication, but with a publication date outside the stated time fi-ame.

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HONORS/AWARDS RECEIVED DURING CONTRACT/GRANT LIFETIME

Include: All honors and awards received during the lifetime of the contract or grant, and any life achievement honors such as (Nobel prize, honorary doctorates, and society fellowships) prior to this contract or grant.

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