Application of a detailed chemical kinetic model to kerogen maturation

May 1, 1992 - Energy Fuels , 1992, 6 (3), pp 318–326. DOI: 10.1021/ef00033a013. Publication Date: May 1992. ACS Legacy Archive. Note: In lieu of an ...
0 downloads 0 Views 1MB Size
Download PDF
318

Energy & Fuels 1992, 6, 318-326

Application of a Detailed Chemical Kinetic Model to Kerogen Maturation Howard Freund Long Range Research Division, Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77252 Received December 19, 1991. Revised Manuscript Received February 21, 1992

The determination of the oil generation kinetics from a given kerogen sample currently involves the assumption that high-temperature short-time laboratory data are equivalent kinetically to the geological conditions of low temperature and long times that existed as oil was generated. This extrapolation from around 400 "C down to roughly 100 "C covers a range in rate of about 14 orders of magnitude. Clearly, one must be fairly confident that the general chemistry that applies at the laboratory conditions also applies at geological conditions. We have modeled the conversion (pyrolysis) of carbonaceous material, such as kerogen, to liquids using a chemical mechanism consisting of elementary kinetic steps. The complexity of the material is handled by treating the material as a stochastic ensemble of molecules, each one consisting of a collection of different chemical functionalities. Taken together, the ensemble reflects the properties of the material and allows a kinetic description of the pyrolysis process. Hence, we can follow the properties of the hydrocarbons as they are produced. Because the model is rooted in fundamental chemistry (Le., elementary reactions), it is believed that the extrapolation of the model rate constants to geological maturation conditions is valid. The results are consistent with the extrapolation of high-temperature laboratory kinetic data These results support the contention that laboratory kinetic data can indeed be used to predict the timinglkinetics of the kerogen maturation process.

Introduction The determination of the oil generation kinetics from a given kerogen sample currently involves the assumption that high-temperature, short time, laboratory data are equivalent kinetically to the geological conditions of low temperature and long times that existed as oil was generated. This extrapolation from around 400 "C down to roughly 100 "C covers a range in rate of about 14 orders of magnitude. Clearly, one must be fairly confident that the chemistry that applies at the laboratory conditions also applies at geological conditions and that no new mechanistic pathways open up. There has been some experimental work at intermediate temperatures. Saxby et al.' observed oil generation between 250 and 300 "C in their one-degree-a-weekheating rate experiment lasting 6 years. Indeed, this is where oil generation would be expected based on an extrapolation of high-temperature kinetics for the kind of kerogen they used. Another report showed experimentally that, at temperatures as low as 209 "C, high-temperature kinetics were still followed.2 However, even at a temperature as low as 200 "C, the kinetics are about 6 orders of magnitude faster in rate than at 100 "C. Sweeney et ala3developed a kinetic model based on phenomenological first-order kinetics and applied it with the known thermal history of the Uinta basin in Utah to estimate oil generation kinetics, The model's conversion predictions, based on the extrapolation of high-temperature kinetics, were consistent with the estimated conversion of samples from the basin. In another study, Sweeney et aL4used Rock Eval and Pyromat kinetics coupled with thermal history models to pre(1)Saxby, J. D.; Bennett, A. J. R.; Corcoran, J. F.; Lambert, D. E.; Riley, K. W. Org. Geochem. 1986,9,69-81. (2)Freund, H.;Kelemen, S. R. AAPG Bull. 1989,73,1011-1017. (3)Sweeney, J. J.;Burnham, A. K.; Braun, R. L. AAPG B d . 1987,71, 967-985. (4)Sweeney, J.; Talukdar, S.; Burnham, A.; Vallejos, C. Org. Geochem. 1990,16,189-196.

0887-0624/92/2506-0318$03.00/0

dict the timing and extent of oil generation in the Maracaiba Basin of Venezuela. Nevertheless, despite these experimental and modeling successes,there still is concern over the applicability of using laboratory data to estimate oil generation kinetics. The extrapolation of kinetics based on a molecular kinetic mechanism would be more reliable and would help to validate the application of laboratory kinetics to maturation conditions. With an interest in geochemical modeling, the application of detailed chemical kinetics to model compounds was recently reported by DominiL5v6 He studied hexane pyrolysis and showed that extrapolation of the high-temperature rate constant was several orders of magnitude higher than that deduced from his detailed chemical kinetic model. He suggested that the current practice of using high-temperature rate data in geochemical modeling could lead to significant errors in timing. The extension of a molecular level approach to very complex material was introduced in a novel way by Klein and co-w~rkers~-'~ in their work on the pyrolysis of asphaltenes and heavy oils and depolymerization. They introduced the concept of stochastic modeling-building an ensemble of molecules based on distribution functions to describe complex carbonaceous material. Klein et al. has coupled this ensemble to stochastic kineticsav9as well as to deterministic kinetics.1° The authors used experimentally derived fit-order rate constants based on model compounds for the kinetic rate constants or transition probabilities. For the deterministic case, the resulting rate

(5)Doming, F.,Org. Geochem. 1991,17, 619-634. (6)Doming, F.;Enguehard, F. Org. Geochem. 1992,18, 41-49. (7)Train, P.M.; Klein, M. T. In Pyrolysis Oils f r o m Biomass; S o h , E. J., Milnes, T. A., Ed.; ACS Symposium Series 376;American Chemical Society: Washington, DC, 1988. (8) Neurock, M.; Libanati, C.; Nigam, A.; Klein, M. T. Chem. Eng. Sci. 1990,45,2083-2088. (9)McDermott, J. B.; Libanati, C.; LaMarca, C.; Klein, M. T. Znd. Eng. Chem. Res. 1990,29,22-29. (IO) Savage, P. E.; Klein, M. T. Chem. Eng. Sci. 1989,44,393-404.

0 1992 American Chemical Society

Energy & Fuels, Vol. 6, No. 3, 1992 319

Kerogen Maturation

Figure 1. A chemical structural model for the Rundle Ramsey Crossing oil shale.'l

equations were integrated using as initial conditions the concentrations determined from the initial distribution functions. To reflect the effects of pyrolysis, these distribution functions were updated and the reaction products determined. Reasonable agreement with experimental data was obtained for quantities like coke make and rate of asphaltene conversion. As there was no fundamental chemistry (i.e., elementary reactions) incorporated into such a model, extrapolation to conditions outside the data base is quite risky. The stochastic modeling concept seemed an excellent way of handling the structural complexity of carbonaceous material. If the stochastic assemblage were coupled to a more rigorous set of kinetics based on elementary chemical reactions rather than phenomenological results, the results could be much more reliably extrapolated to very low temperature. Such an extrapolation would help to answer the question of whether the high-temperature chemistry global kinetics also applied at geological conditions. In this report, the combination of using stochastics to describe the kerogen and elementary reactions to describe the pyrolysis chemistry will be presented. First, the technique of building a stochastic ensemble to describe complex material will be described. Then, a detailed chemical kinetic model will be developed for the laboratory pyrolysis of Green River oil shale using TGA data. Finally, it will be applied to maturation by changing to an appropriate time-temperature history. To begin development of the model, a description of the kerogen is needed which will allow the tracking of reactive moieties through the conversion.

Stochastic Ensemble Assembly Figure 1 represents the model of Australian Rundle kerogen by Scouten et al." Rundle oil shale is a Type I organic matter type and is similar in chemical structure to Green River kerogen. The material can be considered to be a set of ring systems (naphthenic, naphthenoaro(11)Scouten,C.G.;Siskin, M.;Rose, K.D.;Aczel, T.;Colgrove, S.5.; Pabst, R. E.,Jr. Prep.-Am. Chem. Soe., Diu. Pet. Chem. 1989,36(1), 43.

matic, and aromatic) or cores connected by alphatic chains with additional chains including methyl groups as pendants. In this work, heteroatoms are replaced by carbon and five-member rings are treated as six. To assemble stochastically material that would look like that in Figure 1, distribution functions of critical characteristics are needed. These are functions which describe the probability of how a particular functional group is distributed throughout the material. Clearly, one distribution function needed is the number of ring systems (or cores) per molecule. For tracking purposes, these cores are assumed to be connected linearly together. A large molecular weight (so00) was assumed so that a molecule-like formalism could still be maintained. The structure shown in Figure 1 has a molecular weight of 30000. Another distribution function needed to construct a molecule is the number of aromatic carbons in the ring system. Similarly, for naphthenic carbon, a size distribution is needed. The number of pendants on a core is also considered. A pendant is defined as an alkyl chain of four or more carbons. Only alkylaromatics with three or more carbons can undergo a long free-radical chain reaction. The carbon chain must be at least four carbons long to have the three kinetic pathways available to it (see chemical kinetics mechanism section). Methyl groups are treated separately (chains less than four carbons would be treated as multiple methyls). Interconnecting links can vary in length from zero (biphenyl type linkage) to a chain of 14 carbons. Total pendant length is also considered as a distribution function. Another key distribution function is the type or nature of the chemical link resulting from the attachment of an alkyl group to a ring system. As in Savage and Klein,lo three kinds of links are considered: alkylaromatic, alkylnaphthenoaromatic, and alkylnaphthenic. In addition, links of carbon length two (bibenzyl-like) are defined as initiators. Distribution functions are ale0 used for the corecore link length (thiswould include the initiators) and the number of methyls on an aromatic or naphthene. For tracking purposes we consider charaderistics of each core so that the total molecule can be followed. For this we need to monitor the kind of carbon (aromatic, na-

Freund

320 Energy & Fuels, Vol. 6, No. 3, 1992 Structures of Aromatics

6

10

14

16

18

A

20

22

Structures of Naphthenoaromatics A

12

8

4

16

12

8 AmmW

carbon 2 16

Figure 2. Aromatic and naphthenoaromatic structures allowed in the construction of a molecular ensemble. For simplicity, the naphthenoaromatic structures are shown with only one aromatic ring. Acceptable structures would include any of the listed aromatic sizes.

phthenic, aliphatic, olefinic, methyl), the size of the molecule, number of ring systems in the molecule, and the types of links in the molecule or core. Because we want to be able to track products as well, we allow for molecules to leave the system by flagging them as products or reactants. Because core properties are kept track of, molecule properties are easily determined. We keep explicit track of the number of carbons. However, the number of hydrogens in the molecule will depend on precisely how the carbons are arranged. Hence, certain constraints are needed as to how molecules are assembled and what structures are allowed. Aromatics and naphthenoaromatic cores are assumed to exist as depicted in Figure 2. The numbers below the structures show the number of aromatic and naphthenic carbon. Many of these specific kinds of structures have been identified in crude oil. Given these constraints, H/C ratios and molecular weights can be calculated for each molecule or the entire ensemble by accounting for the number of substituents and the number of links to other cores. To assemble a collection of molecules, an ensemble is generated from an integrated probability distribution. The structure in Figure 1served only as a rough guideline for developing the distribution functions. For example, the aromatic size distribution was taken to be as follows, where N represents the number of aromatic carbons in the ring system: % of cores

10 26 24 14

Chemical Kinetic Mechanism The detailed chemical kinetic package is rooted in the author's work on butylbenzene.12 In that work, H atom elimination reactions from alkyl radicals (e.g., ethyl ethylene + H) were not important at temperatures less than 500 "C and at liquid-phase concentrations. In an effort to minimize complexity, reactions producing H atoms from alkyl radicals were omitted. The three pathways for the pyrolysis were based on three different radicals which could arise on abstraction of a hydrogen from the parent: the stabilized benzylic radical, the radical centered three carbons away from the aromatic ring which has a very facile &cleavage pathway (?-radical), and all other secondary radicals which are considered kinetically equivalent. These secondary radicals were lumped into a new species. The same intrinsic rate constants were used for H transfer and &elimination as in the butylbenzene model, although the increased number of hydrogens available for abstraction were included in the applicable rate constants. The terminal primary radical at the end of the alkyl chain, although less stable than the secondary radicals by 2 kcal/mol, was lumped in with the other secondary radicals to keep the number of species a workable number. The interconversion of the three radicals via intramolecular H transfer was treated as 1,4 and 1,5 H-transfer steps. Recently, the mechanism for the pyrolysis of 1,20-di(lpyreny1)eicosane was rep~rted.'~Pyrolysis of thismaterial required the introduction of a new pathway in the pyrolysis to account for direct side-chain cleavage at the ring (reactions 86-91 in Table I of the Appendix). In addition, termination rate constants were reduced significantly to account for the decreased encounter efficiency of two very large radicals. For kerogen, more complex chemistry is needed to account for naphthenoaromatic molecules which are dominant moieties in natural carbonaceous materials. The moiety alkylnaphthenoaromatic was introduced using an alkyltetralin, and associated radical species (again the secondary radicals resulting from H-atom abstraction down the alkyl chain were lumped together). For the purpose of the mechanism, 2-ethyltetralin was used. This compound's pyrolysis has been discussed in Savage and Klein.14 The decomposition pathway for this species led to the dehydrogenation of the ring to form naphthalene (and

-

Ammw carbon c 16

4

were added to allow residue or pyrobitumen formation as maturation or thermal conversion proceeds. This was done as no simple pathway to residue could be determined based on Figure 1 without them. Because molecular properties of all molecules are tracked, it is easy to determine molecular properties of the ensemble after it is built up. For the Green River-like material, we obtained H/C = 1.48. fraction aromatic carbon = 0.29, fraction paraffinic = 0.517, and fraction naphthenic = 0.193. The H/C for Green River shale used in the kinetics experiments is 1.57 and the aromaticity is 0.26. The model values, while not matched exactly to the real material, are close enough for a kinetic examination of the material and to answer the question of whether high-temperature kinetics extrapolate to very low temperatures.

N 0 6 10 14 16

10 8

18

6 2

20 22

Although no large ring systems (greater than 18 carbons, four rings) are evident in Figure 1, some large aromatics

(12)Freund, H.;Olmstead, W. N. 2nt. J. Chem. Kinet, 1989, 21, 561-574. (13)Freund, H.;Matturro, M. G.; Olmatead, W. N.; Reynolds, R. P.; Upton, T. H. Energy Fuel 1991,5, 840-846. (14)Savage, P. E.;Klein, M. T. 2nd. Eng. Chem. Res. 1988, 27, 1348-1356.

Energy & Fuels, Vol. 6, No. 3, 1992 321

Kerogen Maturation alkylnaphthalene). This introduced H atoms which were allowed to abstract H to become molecular H2 as well as to attack atkylaromatics at the ipso position, thus denuding the aromatic. To account for naphthenoaromatic in general, the moiety tetralin and the two radicals resulting from H atom abstraction were also introduced. Appropriate abstraction and termination reactions were added. Although additional species and reactions were added, no new kinds of reactions were considered. The same chemistry is occurring and the reactions added were of a similar type as already in the model (H-abstraction, 6-elimination, and termination). Aromatic structure affects the stability of the moieties rather strongly in the case of benzylic species. The C-H bond leading to the stabilized benzylic radical decreases 3 kcal going from alkylbenzene to alkylnaphthalene and 8 kcal15 going from alkylbenzene to alkylpyrene. In our model we use an average value of 4 kcal relative to alkylbenzene (this is equivalent to an average ring size just over 3). To account for coke formation or pyrobitumen, we added a large aromatic moiety of five rings or more as a reactant. This material was considered to be quite reactive toward further molecular weight growth via addition of a radical to the aromatic ring. The adduct was defined as a coke precursor. The thermochemistry of reactive species is important because the reverse rate constants for reversible reactions are calculated using the equilibrium constant and the forward rate constant given in the mechanism. The equilibrium constant (via the Gibbs free energy) is determined by the thermochemistry of the species involved. Seven reactive moieties were used in the kinetics mechanism (the specific molecule used to calculate thermodynamic properties and for material balance is shown parenthetically): alkylaromatic (butylbenzene), alkylnaphthenoaromatic (2-ethyltetralin), alkylnaphthene (butane), naphthenoaromatic (tetralin), methylaromatic (toluene), initiators (bibenzyl), and the aforementioned large aromatic (methylnaphthalene). Initial concentrations are first counted in the ensemble, normalized, and sent to an integrator as mole fractions. The integrator used is LSODE, and the kinetics package is C H E M K I N . ~ ~With the initial concentrations determined, the integrator is then allowed to calculate up to a few percent conversion using the kinetic mechanism. Concentrations of products which come from certain bond cleavage reactions are used to update the ensemble for the particular chemistry involved. For example, alkylaromatics yield a-olefins as a major product (propylene in the model). The value of the mole fraction of this particular product divided by the mole fraction of the starting moiety determines the fractional amount of that particular moiety which has reacted. This value is then sent to an update program. Updating the Ensemble. In the update program a random core is queried looking for the appropriate functionality, say alkylaromatic. When it finds one, the program randomly determines whether to break a core-core link or a corependant link depending on what is available. The core characteristics are updated to reflect bond cleavage. Correspondingly, the alkyl part is assigned to a product molecule if it was an alkyl pendant (cleavage of alkyl pendants is assumed to always lead to products) or it becomes an alkyl pendant on the neighboring core. The new species are then queried as to whether they would be (15) Stein, S. E.; Golden, D. M. J. Org. Chem. 1977, 42, 839-841. (16)(a) Kee, R. J.; Miller, J. A.; Jefferson, T. H. Sandia Laboratories, SAND 80-8003; (b) Hindmarsh, A. C. LSODE, ordinary differential equation solver; Lawrence Livermore Laboratory, 1980 version.

time (SIC) 600

0

1200

1800

2400

3000

- - - - - - - \o 02-

3600

4200

____~

\c

10 5 % restdue

Ilns hom hgh temp

1616-

win11 liom flcchaitiC

mode1

2-

Figure 3. log conversion vs time a t T = 427 represents the experimental rate constant.

OC.

The line

light enough to leave the system. The boiling point (bp) of the molecule is used to determine whether it stays in the reactor or not. The bp is determined from H/C and molecular weight. The cut point or escape temperature of the TGA has been estimated to be about 1150 O F . Hence, if the boiling point of a molecule is determined to be less than this cut point, it is removed from the system as a product. The process is repeated for the other productalpathways which are represented in the mechanism. The updated ensemble is queried again and concentrations of the reactive moieties are determined. For the purposes of the integrator, before each iteration, the stable products are set equal to zero. The reactant concentrations, including the radicals from the previous calculation, are renormalized and sent down to the integrator for another few percent conversion. This process is repeated until the total reaction time has been used. Conversion is determined from the ensemble by ratioing the mass of the products to the starting material. Model Development for High Temperature. The global kinetics for the pyrolysis of Green River kerogen are ~ell-known.~J~ Values measured in a TGA reactor [k = 1.00 X 1013 exp(-51 300/RT)s-'I2 corresponding to oil generation were used and were consistent with other published data.17 The mechanism discussed above was used for the pyrolysis and is reproduced in the Appendix (Table I). Most of the rate constants for the elementary reactions are known or can be reliably estimated. However, there is uncertainty in a number of rate constants and these were considered parameters and optimized by the model. For example, the rate constant for the addition of a radical into a five-ring aromatic system to form a coke precursor (reaction 163 in Table I) is uncertain and was determined by fitting the conversion to the high-temperature data. A structural parameter, the number of ring systems linked directly together with no carbons between them, was one of the factors which determined the amount of residue (this would be pyrobitumen or coke). Coke precursors (high molecular weight material with no long side chains and few naphthenic rings) could be formed directly by the chemistry. In addition, material left at the end of the reaction was assumed to end up as coke. Cyclization of alkyls and condensation reactions were not included. Another structural parameter determined the number of initiators in the system. For the Green River kerogen only one type of initiator was used. Optimizing these kinetic and structural parameters allowed a match between experiment and model to be obtained at high temperature (400-500 "C). (17) Campbell,J. H.; Koskinas, G. J.; Stout,N. D. Fuel 1978,57, 217.

322 Energy & Fuels, Vol. 6,No. 3, 1992 --

0.9

--

0.8

I

-- 0.7 6 -- 0.8 -- 0.5

---

‘ --.--* A 0 , 2 1

-10

.

-I5

30

35

i i

-

-

00012

OW14

-Sle*u*r

Um* R W l *

“C

441%

OW16

OW18

182%

OW2

1K (1IK)

OW-

OW24

OW26

Figure 5. Arrhenius plot of the global rate constant. The solid line represents the experimental value while the points are from the kinetic model.

Figure 3 shows a plot of the log of the conversion vs time at T = 427 OC. Although there clearly is some oscillation in the model results about the experimental line, reasonable agreement was obtained. The amount of residue as determined from the model is 10.5%. Experimentally, the value is about 12%. Extrapolation to Geological Conditions. Isothermal Cases. For the maturation of kerogen, the system is considered closed, i.e., products remain in the “reactor”. The code was altered so that all products except methane were kept in the reactor and further chemistry was allowed to occur on these products. For the purpose of calculating conversion, however, the high-temperature definition was used-material boiling below 1150 O F was considered product. Products will have an “open system” look as they contain olefms. It is secondary reactions which lead to the saturation of the olefinic linkage with the hydrogen for this coming from the residual kerogen. This secondary chemistry only minimally perturbs the primary bond-breaking reactions described in the mechanism and is not considered. With the model fit to high-temperature data, it was applied to the isothermal maturation of kerogen. No kinetic or structural parameters were changed, but the nature of the reador was changed as discussed above. Figure 4 shows results at T = 127 O C . The line is a best fit through the model points. Its slope is the first-order rate constant at these conditions. Note that the amount of residue has now increased to 15%. This increase is expected since the system is now closed and liquid products can further react and be converted to coke. If the system is made open, i.e., the exact model from the high temperature case is used but the temperature is lowered, the kinetics come out similarly but the amount of residue is closer to 10.5%.

surmm*.pan”

-

m

Figure 4. log conversion vs time at T = 127 “C. The line is a linear least-squares fit to the computer model results. The slope of the line is the global first-order rate constant.

-

t

3

a

o

m

4

2

0

4

4

0

4

8

0

4

e

4

5

0.4

1 f

0.3 0 0.2

o‘ 0

M

E W E R I N R E (‘0

Figure 6. Nonisothermal results for the high heating rata case. The lines are the rate of generation and cumulative generation for the case of two parallel global first-order reactions, El and E2. The points are determined from the kinetic model.

Figure 5 is an Arrhenius plot of the high-temperature data together with the rate constant (diamonds) obtained from Figure 4. Also included is another point (squares) at low temperature, calculated using a more aromatic kerogen. Such material would be representative of a more marine kerogen. We wanted to examine the sensitivity of the system to aromaticity since one of the sensitive reactions in the mechanism was the cleavage reaction of a large-ringed alkylaromatic. This is a hypothetical comparison as the phenomenological rate constant is assumed not to change with increased aromaticity. The point comes out slightly higher than the extrapolated curve but is still within 5-6 O C of the line. Clearly the model is sensitive to aromaticity but it is a small effect. The model results are quite consistent with first-order kinetics and cover a wide range in temperature. This work has shown that, for the case of the k i n e t i d y homogeneous Green River kerogen, these reactions together can be fit to a first-order Arrhenius expression with a global activation energy. It is this global rate constant which is valid over a wide temperature range. Nonisothernuil Cases. Most kerogens have considerably more kinetic complexity than the well-behaved Green River kerogen. This is due to a distribution of initiators with different activation energiea which broaden the overall distribution. The phenomenological rate expression for this more general system can be described by a set of parallel fmt-order reactions with the same preexponential factor but different activation energies. A simple way of extending the work on the Green River kerogen to a more complex system is to imagine that the kerogen has more than one initiator present. Although there is no experimental data for this hypothetical system, the behavior being modeled at high temperatures is that produced by two parallel first-order reactions. Nonisothermal kinetics are used to describe the system. The issue is whether the system, with the addition of a second initiator, is still adequately described phenomenologically by parallel fmt-order reactions and whether these rate constants can be widely extrapolated in temperature. Figure 6 shows both the generation rate as well as the cumulative generation. The lines result from the sum of two fmt-order reactions with activation energies 50.8 (25% contribution) and 52 (75% contribution) kcal/mol. These are the hypothetical experimental results. The detailed chemical kinetic model results are represented by the points. This is the nonisothermal analogue to the hightemperature isothermal case. Here, the heating rate (10 deg/min) is similar to typical Rock-Eval laboratory heating rates (25 deg/min). The points in Figure 6 are considered close enough to the lines for a reasonable fit. The model

Energy & Fuels, Vol. 6, No. 3, 1992 323

Kerogen Maturation *--*--*-4

-

El- So 8 Wmb A-31E13-

T

1

1

0 9

OB

07 06

el

90

1W

I10

IHI

130

140

1%

1

160

TEMFWMTURE

Figure 7. The nonisothermal

case a t a geological heating rate.

contains two initiators, one with an activation energy of 48 and the other a value of 52 kcal/mol. The concentration of the 52 kcal/mol initiator is initially twice that of the 48 kcal/mol species. For maturation, the heating rate is slowed to 0.6 deg/ million years (2E-14 "C/s). Figure 7 shows the model results. The agreement with the rate of generation plot is not particularly good since the peak maxima do not line up. There appears to be an offset of about 5 deg. In the cumulative generation plot, the offset appears to be only about 3 deg. However, because this small offset is essentially equivalent to the experimental uncertainty in the high-temperature rate constants, the kinetic model with two initiators is still reasonably well described by two phenomenological rate constants. These results support the general statement that, for kerogen transformations, parallel first-order rate constants obtained at high heating rates in the laboratory can indeed be extrapolated to maturation conditions.

Discussion The problem of oil generation from kerogen (thermal conversion) is an incredibly complex process because of the structural complexity of the kerogen. Clearly, assumptions have to be made to make the problem tractable. In this work, there are three issues: (1) the structural representation of the kerogen in terms of chemical moieties; (2) the kinetic representation of the elementary reactions needed to describe the thermolysis of (1);and (3) the evolution of (1) by (2) as conversion proceeds. Structurally, we have included a collection of chemical functionalitiesbelieved to be in kerogen. Although we have made a number of simplifying assumptions about how the material is put together, features such as overall aromaticity and H/C content are approximately matched to real materials. The kinetic mechanism consists of a collection of elementary first- and second-order reactions describing mostly well-known free-radical chemistries. An important assumption is that the moieties responsible for these chemistries react as if they are independent and that being bound to the rest of the kerogen does not affect this. Another key assumption is that approximately equivalent kinetic and thermodynamic species/functionalities can be lumped into one species. Kinetic pathways such as condensation and cyclization reactions are not considered. With these kinds of simplifying assumptions, the conversion of kerogen to products can be modeled at laboratory conditions. Given such a fundamentally based model, the extrapolation to geological conditions is more sound. It does appear that global kinetics can indeed describe the model over a wide temperature range. This by itself is interesting since one is dealing with a system containing seven reactive functionalities and a reaction mechanism consisting of 171 unimolecular and biomolecular reactions

described by a wide range of kinetic rate constants. Although the activation energy does not change from 527 to 127 "C, there are changes in the mechanism. At 527 "C, H atom ipso attack (e.g., reaction 68) is more than a factor of 2 slower than hydrogen abstraction (e.g., reaction 67), while at 127 "C the situation is reversed. This means that, at low temperature, H atoms will tend to liberate alkyl radicals. The H atoms are produced by ring dehydrogenation reactions. As Doming has reported: the high-energy unimolecular &elimination reactions (reactions 159-163) are essentially shut down at low temperature. In this work, however, there exists a facile, low-energy &elimination reaction of the alkylaromatic y-radical to produce propylene and the benzyl radical (reaction 158) and this becomes one of the dominant carbon-carbon bond breaking reactions. If there is a low-energy decomposition channel in an alkyl system, the radical, unable to decompose along any other channel, will effectively shuttle down the alkyl chain until it finds it. These model results differ with those of D ~ m i n g . He ~,~ developed a detailed chemical kinetic mechanism for the pyrolysis of hexane and observed a retardation of the overall rate constant with temperature k e a ,an increase in the activation energy of the overall reaction). In hexane pyrolysis at high temperature, hexyl radicals readily decompose (@-scission)to an olefin and smaller radicals. If one of these small radicals recombines with hexyl as one of the primary termination steps, the system will be first order.18 Experimentally, Doming reports first-order data obtained in the temperature range 427-500 "C having an activation energy of 61 kca16 As the temperature is lowered, the mechanism changes since hexyl radicals increase in concentration because the hexyl &scission decomposition is a relatively high energy process. Eventually these will become the dominant radicals and their recombination the dominant chain termination step resulting in half-order kinetics. Half-order kinetics were reported by Doming for the temperature range 305-365 "C with an activation energy of 72 kcal/m01.~ In this work, because kerogen consists of many functionalities, the mechanism tracks the decomposition of several species. Some of the termination steps have been heavily attenuated to account for the decreased encounter efficiency of very large radicals. This tends to increase the overall radical concentrations at all temperatures. The dominant radical, i.e., the radical present in highest concentration, is the tetralyl radical (PHCYCCCC) at all temperatures studied. However, the alkylaromatics continue to decompose at low temperature through the lowenergy channel. The alkylnaphthenoaromatics de&late (reactions 84-86) rather than undergo @-scissionreaction 162 or 163. The alkylnaphthenes do not readily cleave at high temperature and are unreactive at low temperature. Although there clearly are changes in the relative concentrations of radicals as a function of temperature, there does not seem to be the kind of change reported by Doming that affects the overall order of the reaction or the global activation energy. The prior discussion applies to the chemical kinetic model. The extension to the real world is leas certain since not only does one have a real kerogen undergoing real conversion, but there could be other factors affecting conversion such as mineral catalysis, water chemistry, or pressure (through activated volume). These factors have not been explicitly account for in this model. (18) Laidler, K. Chemical Kinetics;Harper 81 Row: New York, 1987; pp 307-314.

324 Energy & Fuels, Vol. 6, No. 3, 1992

Freund

Table I. Chemical Kinetic Mechanism reactions considered DreexD _____~ 6.00D+12 1. PHC' + CCC' = alkaromat 2.00D+10 2. H PHC'CCC = PHC*CCC + H2 l.llD+13 3. PH2CCPH - PH2C' + PHC' l.llD+13 4. PHCCPH - PHC1' + PHC1' 3.92D+15 5. PHCYC4'2 - PHCYC'CCC + PHCYC'CCC 1.74D+15 6. PHCYC'BBZ - PHC'CCC + PHCYC'CCC 5.00D+16 7. CCCC - CC' + CC'(Xl0) 3.00D+06 8. PHCYC'CCC + PHCYC'CCC - PHCYC4'2 3.00D+06 9. PHC'CCC + PHCYC'CCC - PHCYC'BBZ 3.00D+06 10.PHC'CCC + PHC'CCC - PHC'CCC2 3.00D+08 11. PHC' PHCYC'CCC - PHCYC4PHC 3.00D+08 12. PHC' + PHC'CCC - PHCCCCCPH 4.00D+09 13. PHCYC'CCC + CC' - TERMCC 4.00D+09 14. PHCYC'CCC + CC1' - TERMCCl 4.00D+12 15. PHCYC'CCC + C' - TERMC 4.00D+12 16. PHC' + C' - PHCC 4.00D+12 17. PHCYCI'CP + C' - TERMlC 4.00D+12 18. PHC'CCC + C' - PHCCCCC 3.00D+10 19. _PHCYC'CCC _ _ _ ._ _ _ _ + CCCC'* _ _ _ _ - TERMC4 ~. 3.00D+08 20. PHCYC'CCC + PHCC' - TERMPHCC 5.00D+15 21. PHC'CCC2 - PHC'CCC + PHC'CCC 2.44D+15 22. PHCYC4PHC - PHC' + PHCYC'CCC 2.93D+12 23. PHC' + alkaromat = PHC + PHCCC'C 7.50D+00 24. PHC' + PHCYCCCCl = PHC + PHCYC'CCC 1.51D+00 25. PHC' + alkaromat = PHC + PHC'CCC 1.47D+13 26. PHC' PHCYCCCC2 = PHC + PHCYCC'CC 2.93D+13 27. PHC' + alkaromat - PHC + PHCCCC'* 4.21D-03 28. PHCCCW + PHC - alkaromat + PHC' 4.21D-03 29. CCCC'* + PHC - PHC' + CCCC 2.90D+13 30. PHC' + CCCC - PHC + CCCC" 7.51D+00 31. PHC' PHCYC4C2 = PHC + PHCYCI'C2 3.00D+00 32. PHCCC'C alkaromat - PHCCCC'* + alkaromat 1.50D+00 33. PHCCC'C PHCYCCCCS = alkaromat + PHCYCC'CC 1.50D+00 34. PHCCC'C + PHCYCCCCl = alkaromat + PHCYC-CCC 3.00D-01 35. PHCCC'C + alkaromat = alkaromat + PHC'CCC 3.00D+00 36. PHCCC'C + CCCC - alkaromat + CCCC'* 1.50D+00 37. PHCCC'C + PHCYC4C2 = alkaromat + PHCYCI'C2 3.00D+00 38. PHCCC'C + PHCYC4C2 - alkaromat + PHCYC4C2' 3.00D-01 39. PHCCCC'* + alkaromat - alkaromat + PHC'CCC 3.00D-01 40. PHCCCC'* + alkaromat - PHCCC'C + alkaromat 3.00D-01 41. PHCCCC'* CCCC - alkaromat + CCCC'* 1.50D+00 42. PHCCCC'* + PHCYCCCCP - alkaromat + PHCYCC'CC 1.50D+00 43. PHCCCC'* + PHCYCCCCl - alkaromat + PHCYC'CCC 1.50D+00 44. PHCCCC'* + PHCYC4C2 - alkaromat + PHCYCI'C2 3.00D+00 45. PHCCCC'I + PHCYC4C2 - alkaromat + PHCYC4C2' 3.00D-01 46. CCCC'* alkaromat - PHC'CCC + CCCC 3.00D-01 47. PHCCC'C ... CCCC'* _ _ _ _ + alkaromat - ~ ~ .~CCCC . . . 3.00D+00 48. CCCC'* + alkaromat - PHCCCC'* + CCCC 1.50D+00 49. CCCC'* + PHCYCCCCl - PHCYC'CCC + CCCC 1.50D+00 50. CCCC'* + PHCYCCCCP - PHCYCC'CC + CCCC 1.50D+00 51. CCCC" + PHCYC4C2 - CCCC + PHCYCI'C2 3.00D+00 52. CCCC" + PHCYC4C2 - CCCC + PHCYCICS' 1.50D+00 53. PHC'C + alkaromat - PHC'CCC + PHCCl 2.50D+12 54. PHC'C + alkaromat - PHCCC'C + PHCCl 2.50D+13 55. PHC'C + alkaromat - PHCCCC'* + PHCCl 1.50D+00 56. PHC'C + PHCYCCCCl - PHCYC'CCC + PHCCl 1.WD+ 13 57. PHC'C + PHCYCCCCP - PHCYCC'CC + PHCCl 1.50D+00 58. PHC'C PHCYC4C2 - PHCYCI'C2 + PHCCl 3.00D-01 59. PHCYC4CS' + alkaromat - PHC'CCC + PHCYC4C2 3.00D-01 60. PHCYC4CP' + alkaromat - PHCCC'C + PHCYC4C2 61. _ PHCYC4C2' + alkaromat - PHCCCC'* PHCYC4C2 3.00D+00 _ _ _ ._ _ _ . - ~~. ... 1.50D+00 62. PHCYCICP' + PHCYCCCCl - PHCYC'CCC + PHCYC4C2 1.50D+00 63. PHCYCICS' + PHCYCCCCS - PHCYCC'CC + PHCYC4C2 1.50D+00 64. PHCYCICS' PHCYC4C2 - PHCYC4C2 + PHCYCI'C2 1.20D+ 14 65. H + alkaromat - H2 + PHC'CCC 1.30D+06 66. H + alkaromat - H2 + PHCCC'C 1.30D+07 67. H alkaromat - H2 + PHCCCC'*(XlO) 3.60D+13 68. H + alkaromat - BZ + CCCC'* 6.00D+14 1.30D+06 6.00D+14 1.30D+07 1.20D+14 74. 3.60D+13 . -. H - - +. PHC - - - - - C* - +. C6H6 - --- 1.00D+ll 75. PHC'CCC - PHCCCC'* (1,5) 1.20D+11 76. PHCCCC" - PHC'CCC (1,4-X0.2) 2.00D+10 77.PHCCCC'* - PHCCC'C (1,4-X0.033) 1.20D+12 78. PHCCC'C - PHCCCC'Z (1,4) 1.00D+ll 79. PHCYCI'C2 - PHCYC4CP' (1,5) 1.20D+11 80. PHCYC4C2' - PHCYCI'C2 (1,4-X02*1) 3.80D+13 81. PHCYC'CCCC + alkaromat - PHCYCCCCl + PHCCCC'. 3.80D+13 82. PHCYC'CCC+CCCC - PHCYCCCCl + CCCC'* 1.50D+00 83. PHCYC'CCC + PHCYC4*C2 - PHCYCCCCl + NAPH + CC1' 1.50D+00 84. PHCYCI'C2 + PHCYC4C2 - PHCYC4C2 + NAPH + CC1' 1.50D+00 85. PHC'CCC + PHCYC4*C2 - alkaromat - NAPH + CC1' 1.00D+ll 86. PHCYC'CCC + alkaromat - BZ + CCCC'* + PHCYC*CCC

+

+

~

+ +

+ +

+

+

+

~~~~

+

+

+

+

~

~~~

~

~

~~~

temD exD 0.000 O.Oo0 O.Oo0 O.Oo0

0.000 0.000 0.000 O.Oo0 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 3.460 3.460 O.Oo0

0.000 4.200 4.200 0.000 3.460 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 3.650 0.000 0.00 3.650 0.000 3.650 3.650 3.650 3.65 3.650 3.650 3.650 0.000 2.400 2.400 O.Oo0

0.000 2.400 0.000 2.400 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 O.Oo0

0.000 3.650 3.650 3.650 0.000

act EM 0 0 52000 48000 52400 52300 85000 0 0 0 0

0 0 0

0 0 0 0 0 0

52400 55700 25700 7470 7470 25700 25700 6710 6710 25700 7470 9140 9140 7140 7140 9140 7140 9140 7140 9140 9140 9140 7140 7140 9140 7140 9140 9140 7140 9140 7140 9140 9140 27785 27705 9140 27600 9140 7140 9140 9140 7140 9140 7140 8000 4470 4470 4120 8000 4470 8000 4470 8000

4120 27600 18000 18000 18000 27600 18000 27600 27600 6140 6140 6140 25000

Kerogen Maturation

Energy & Fuels, Vol. 6, No. 3, 1992 326

Table I (Continued) reactions considered 87. PHCYCI'C2 + alkaromat - BZ + CCCC'* + PHCYC4*C2 88. PHC'CCC + alkaromat - BZ + CCCC'* + PHC*CCC 89. PHCYC'CCC + pol- - NAPH* + C' + PHCYCICCC 90.PHCYCI'C2 pol- - NAPH* + C' + PHCYC4*C2 91. PHC'CCC + p o b - NAPH* + C' + PHCICCC 92. PHCYC'CCC + PHC*CCC = PHCYC*CCC + PHC'CCC 93. PHCYC'CCC + PHC*C = PHCYC'CCC + PHC'C 94. PHCYCC'CC - PHCYCZCCC + H 95. PHCYCC'CC - PHCYC*CCC + H*' 96. PHCYC'CCC - PHCYCICCC + H 97. PHCYC'CCC - PHCYCICCC + H*' 98. PHC'CCC + PHCYC4C2 = alkaromat + PHCYCI'C2 99. PHC'CCC + PHCYC4C2 - alkaromat + PHCYC4C2' 100. PHCYCI'C2 alkaromat - PHCYC4C2 + PHCCCC" 101. PHCYCI'CO PHCYC4C2 - PHCYC4C2 + PHCYC4C2' 102. PHCYCI'C2 + CCCC - PHCYC4C2 + CCCC'* 103. PHCYCI'C2 PHCYCCCCl = PHCYC4C2 + PHCYC'CCC 104. PHCYCI'C2 + PHCYCCCC2 = PHCYC4C2 + PHCYCC'CC 105. PHCYC'CCC + PHCYC*CCC = PHCYCCCCl + PHCYC4*' 106. PHCYC*CCC + PHC'CCC = PHCYC4*' + alkaromat 107. PHCYCI'C2 + PHCYC*CCC = PHCYC4*' + PHCYC4C2 108. PHCYCC'CC + alkaromat - PHCYCCCCS + PHCCCC'* 109. PHCYCC'CC + PHCYCCCCl = PHCYC'CCC + PHCYCCCCS 110. PHCYCC'CC + PHCYC4C2 - PHCYCCCC2 + PHCYCICS' 111. PHCYCC'CC + alkaromat = PHC'CCC + PHCYCCCCl 112. PHC'CCC + alkaromat - alkaromat + PHCCCC'* 113. PHC'CCC + PHCYCCCCl = alkaromat + PHCYC'CCC 114. CC' + alkaromat - CC + PHCCC'C 115. CC' + alkaromat - CC + PHC'CCC 116. CC' + alkaromat - CC + PHCCCC'* 117. CC' + CCCC - CC + CCCC" 118. CC' + PHCYCCCCl - CC + PHCYC'CCC 119. CC' + PHCYCCCCP - CC + PHCYCC'CC 120. CC' PHCYC4C2 - CC + PHCYCI'C2 121. CC' + PHCYC4C2 - CC + PHCYC4C2' 122. CC' + PHC - PHC' + CC 123. CC1' + alkaromat - CC1 + PHCCC'C 124. CC1' + alkaromat - CC1+ PHC'CCC 125. CC1' + alkaromat - CC1+ PHCCCC'. 126. CC1' + CCCC - CC1 + CCCC" 127. CC1' + PHCYCCCCl - CC1+ PHCYC'CCC 128. CC1' + PHCYCCCCP - CC1+ PHCYCC'CC 129. CC1' + PHCYC4C2 - CC1+ PHCYCI'C2 130. CC1' + PHC - PHC' + CC1 131. C' + alkaromat - C + PHCCC'C 132. C' + alkaromat - C + PHC'CCC 133. C' + alkaromat - C + PHCCCC'* 134. C' + CCCC - C + CCCC'* 135. C' PHCYCCCCl - C + PHCYC'CCC 136. C' + PHCYCCCCS - C + PHCYCC'CC 137. C' + PHCYC4C2 - C + PHCYCI'C:! 138. PH2C' + alkaromat - PH2C + PHC'CCC 139. PH2C' + alkaromat - PH2C + PHCCC'C 140. PH2C' + alkaromat - PH2C + PHCCCC'I 141. PH2C' + PHCYCCCCl - PH2C + PHCYC'CCC 142. PH2C' + PHCYCCCC2 - PHPC + PHCYCC'CC 143. PH2C' + CCCC - PH2C + CCCC'* ( X l O ) 144. PH2C' + PHCYC4C2 - PH2C + PHCYCI'C2 145. PHC1' + alkaromat - PHCl + PHC'CCC 146. PHCI' + alkaromat - PHCl + PHCCC'C 147. PHC1' + alkaromat - PHCl + PHCCCC'. 148. PHC1' + PHCYCCCCl - PHCl + PHCYC'CCC 149. PHC1' + PHCYCCCCS - PHCl + PHCYCC'CC 150. PHC1' + CCCC - PHCl + CCCC'* 151. PHC1' + PHCYC4C2 - PHCl + PHCYCI'C2 152. PHCC' + alkaromat - PHCC + PHC'CCC 153. PHCC' + alkaromat - PHCC + PHCCC' 154. PHCC' + alkaromat - PHCC + PHCCCC'* 155. PHCC' + CCCC - PHCC + CCCC" 156. PHCC' + PHCYCCCCl - PHCC + PHCY 157. PHCC' + PHCYCCCCS - PHCC + PHCYCC'CC 158. PHCC' + PHCYC4C2 - PHCC + PHCYCI'C2 159. PHCC + PlICYC4C2 - PHCC + PHCYC4CP' 160. PHCC + PHC - PHC' + PHCC 161. PHCYC4" - NAPH + H 162. PHCYC3*' - NAPH + H" 163. PHCYC'CCC + polar8 - precoke + C' 164. PHCCC'C - PHC' + C*CC 165. PHC'CCC - PHC*C + CC' 166. PHCCCC'* - PHCC' + C*C 167. CCCC'* - CC' + C*C2 168. PHCYC4'CP - PHCYC*CCC + CC1' 169. PHCYCICP' - PHCYCC'CC + C*C1 170. H*' + alkaromat - BZ + CCCC'* 171. HI' + PHC - BZ + C'

+

+ + +

+

+

preexp 1.00D+ll

temp exp 0.000

0.000 0.000 0.000 0.000

0.000 0.000 0.000 0.000

0.000 0.000

3.650 0.000

0.000 0.000 0.000

3.650 0.000 3.650 3.650 3.650 3.650 3.650 3.650 3.650 0.000 3.650 3.460 3.460 3.460 3.460 3.460 3.460 3.460 3.460 3.650 3.460 3.460 3.460 3.460 3.460 3.460 3.460 3.650 3.460 3.460 3.460 3.460 3.460 3.460 3.460 3.460 0.000 0.000 3.460 0.000

0.000 3.460 3.460 0.000 0.000 3.460 0.000 0.000 3.460 3.460 3.460 3.460 3.460 3.460 3.460 3.460 3.460 3.650 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.00

act eng 25000 25000 25000 25000 26000 2oooo 2oooo 38000 29000 49000 4oooo 9140 27785 27600 27600 27600 9140 27600 6140 6140 6140 9140 7140 9140 7140 27785 9140 7470 5470 7470 7470 5470 7470 5470 7470 7140 7470 5470 7470 7470 5470 7470 5470 7140 5480 4480

5480 5480 4480 5480 4480 8400 27785 27785 8400 27785 27705 8400 8400 27785 27785 8400 27785 24785 8400 5470 7470 7470 7470 5470 7470 5470 7470 7140 26000 2oooo 3oooo 11300 38250 28000 26000 38000 26000 0 0

326

Energy & Fuels 1992, 6, 326-328

Doming’s results are valid for light hydrocarbons. This work has examined kerogen. The behavior of generated oil requires further work to determine into which regime it will fall. Summary We have modeled the conversion (pyrolysis) of carbonaceous material, such as kerogen to liquids, using a chemical mechanism consisting of fundamental kinetic steps. The complexity of the material is handled by treating the material as a stochastic ensemble of molecules, each one consisting of a collection of different chemical functionalities. Taken together, the ensemble reflects the properties of the material and allows a kinetic description of the pyrolysis process. Hence, we can follow the properties of the hydrocarbons as they are generated. Because the model is rooted in detailed chemical kinetics based on elementary reactions, it is believed that the extrapolation of the model rate constants to geological maturation conditions is valid. The results are consistent with the extrapolation of global high-temperature laboratory kinetic data and support the idea that laboratory kinetic data can indeed be used to model the timinglkinetics of the kerogen maturation process. Acknowledgment. I thank Drs. Erik Sandvik, Tony Dean, and Bill Olmstead for their suggestions and encouragement of this program. Appendix The reactions considered are listed in Table I. The rate constanta are given in the form k = AT’ exp(E,/RT). The units of the A factor are either s-l for unimolecular reactions or cm3/(mol s) for bimolecular reactions. The A factor is expressed in Fortran format, D9.2. The activation energy, E,, is in kcal/mol. The nomenclature is built around butylbenzene (Alkaromat). Hydrogens are left out

for simplicity unless actually produced as H atoms. PH stands for phenyl group (PHC is toluene); a dot (*) in a species usually indicates a radical; * refers to either a double bond (e.g., C*C) or an indication that there is a lumping of many species. In a long-chain alkane, the secondary internal radicals are essentially all kinetically equivalent. These would be indicated as CCCC’*. BZ and NAPH refer to benzene and naphthalene. PHCYCCCC refers to a naphthenoaromatic species-it is considered tetralin in this analysis. PHCYC4CB is an alkylated naphthenoaromatic (ethyltetralin). PHCl and PHC2 are two different aromatic methyl species to reflect different C-H bond strengths. Reactions 8-20 involve production of a termination product-two radicals recombining to form a product. POLARS is the name given to the five-ring or greater aromatic species and PRECOKE refers to a species considered to be a coke precursor. The equals sign (=) signifies the reaction is reversible and hence the program will calculate the reverse rate constant; a dash (-) signifies an irreversible reaction. Reactions 1 and 2 are from Allara;lg (3) and (4) are initiators with values determined by the model; (5)-(7) and (211422) are bond dissociation rate constants calculated from a conventional recombination rate constant (such as 1)and the species thermochemistry; (8)-(20) are this author’s estimates of reduced recombination rate constants; (23)-(67), (69)-(73), (81)-(85), and (98)-(160) are Htransfer reactions from Tsang;20 (68) and (74) are from Robaugh and Tsang;2l(75)-(80) are the author’s estimates of 1,4 and 1,5 H-transfer global reactions; (86)-(91) are from ref 13; (92)-(93) and (161)-(162) are this author’s; (94)-(97) are from Allara; (163) is a coke-producing reaction whose value is optimized by the model; (164)-(169) are elimination reactions from Allara. (19) Allara, D. L.; Shaw, R. J. Phys. Chem. R e f . Data 1980, 9, 523. (20) Tsang, W. J. Phys. Chem. Ref. Data 1988. (21) Robaugh, D.; Tsang, W. J . Phys. Chem. 1986, 90,4159.

C‘ommuntcatsons Temperature-Programmed Liquefaction of a Low-Rank Coal

Sir: Recently there has been increasing interest in finding ways to improve conversion of low-rank coals such as subbituminous coal and lignite, which are often less readily liquefied than bituminous For a given reaction system, controlling the conditions is important for max(1) Song, C.; Hanaoka, K.; Nomura, M. Fuel 1989, 68, 287. (2) Sorg, C.; Nomura, M.; Hanaoka, K. Coal Sci. Technol. 1987, 11, 239. (3) DOE COLIRN Panel. “Coal Liquefaction”, Final Report, DOEER-0400, Vol. I & 11, 1989. (4) Masunaga, T.; Kageyama, Y. Froc. 1989Int. Cof. Coal Sei., Tokyo, 1989, 819. (5) Derbyshire, F. J.; Davis, A.; Epstein, M.; Stansberry, P. G. Fuel 1986,65, 1233. (6) Derbyshire, F. J.; Davis, A.; Schobert, H.; Stansberry, P. G. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1990, 35, 51. (7) Burgess, C. E.; Schobert, H. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1990, 35, 31. (8)Stenberg, V. I.; Gutenkurt, V.;Nowok, J.; Sweeny, P. G. Fuel 1989, 68, 133.

0887-0624/92/2506-0326$03.00/0

imizing the yield and quality of products and minimizing retrogressive reactions. The temperature-programmed liquefaction (TPL) described here seeks to efficiently liquefy low-rank coals by controlling the rate of pyrolytic cleavage of weak bonds while minimizing the retrogressive crosslinking of radicals and thermally sensitive groups. A Montana subbituminous coal obtained from the Penn State Coal Sample Bank (DECS-9, PSOC-1546) was used after drying in a vacuum oven at 95 “C for 2 h. Liquefaction was carried out in 25-mL microautoclaves using 4 g of coal (C60 mesh) and 4 g of H-donor tetralin solvent under 6.9 MPa of H2. After the reaction, the liquid and solid products were separated by sequential extraction with hexane, toluene, and THF. The THF-insoluble residues were analyzed by 13C NMR using the cross-polarization and magic-angle-spinning (CPMAS)gtechnique, and by pyrolysis-gas chromatography-mass spectrometry (PyGC-MS) (pyrolysis at 610 “C for 10 s ) . ~ (9) Hatcher, P. G.; Wilson, M. A.; Vassallo, A. M.; Lerch 111, H. E. Int. J . Coal Geol. 1989, 13, 99.

0 1992 American Chemical Society