Energy & Fuels 1987, I ,
488
x e
wavelength of radiation, A diffraction angle, deg
Subscripts n component n S internal standard Acknowledgment. We are indebted to E. Yildirim of CanOxy Ltd. for the shale samples and to D. Ball of 3-D
488-496
Geoconsultants, Fredericton, NB, Canada, for their selection and identification. The invaluable advice on XRD work provided by J. Vahtra of the Geology Department at the UNB, and the cooperation of H. DeSouza from the SEM unit at the UNB are deeply appreciated. S.K. was funded by an NSERC Undergraduate Summer Research Scholarship during this work. Registry No. SOz, 7446-09-5; calcite, 13397-26-7;dolomite, 16389-88-1.
Thermodynamic Modeling of Oil Shale Pyrolysis J. M. Charlesworth Materials Research Laboratories, Department of Defence, Ascot Vale, Victoria, 3032 Australia Received June 15, 1987. Revised Manuscript Received August 24, 1987 Theoretical calculations on the high-temperature stability of a range of compounds have been performed and the results compared with the experimentally measured composition of pyrolysis products from model compounds and that of shale oil derived from type I and type I1 kerogen. Acceptable agreement is found for a range of isomers of aromatic and heteroaromatic compounds; however, evidence is found indicating that kinetic factors play a role in determining several isomer distribution patterns. The mineral matter in Rundle shale ash assists in promoting equilibrium, as also does a long residence time at high temperature. Large-scale model calculations allowing for elemental balance and unrestricted reaction between species have so far not predicted many of the most characteristic features of the composition of shale oil. Structural units present in the original kerogen molecule appear to be preserved in the pyrolysis products to a significant extent following retorting at 5 "C min-l. Introduction In order to understand the relationship between shale oil and kerogen, it is desirable that the thermally induced reactions occurring during pyrolysis be characterized. With this aim in mind, theoretical calculations on the likely stability of organic compounds at the temperatures involved in oil shale retorting could prove valuable and should assist in the optimization of retorting conditions to enable the most favorable product distribution to be achieved. Thermodynamic calculations have long been used in the petroleum refining industry to predict the composition of gasoline range material produced by high-temperature cracking of crude oi1.l In this process it is accepted that a network of chemical equilibria is established and that alkyl aromatics are present in ratios determined by their free energies of formation.2 It is observed that reactions producing low molecular weight compounds via the decomposition of high molecular weight alkanes, naphthenes, and aromatics containing long-chain alkyl side groups are the thermodynamically probable ones. From the free energy viewpoint cracking reactions should therefore be most favorable at high temperatures; however, kinetic factors may prevent the attainment of equilibrium. Thermodynamic modeling studies have been applied in a limited way to a study of the composition of shale oil n a ~ h t h ain, ~which it was shown that the experimentally (1) Stull, D. R.; Westrum, E. F., Jr.; Sinke, G. E. The Chemical Thermodynamics of Organic Compounds J . Wiley and Sons, New York, 1969. (2) Draeger, A. A.; Gwin, G. T.; Leesemann, C. J. G.; Morrow, M. R. Pet. Refin. 1951, 30, 71.
0887-0624/87/2501-0488$01.50/0
measured distribution of the C2- and C3-substituted benzenes could be predicted by using free energy data. Digital computer methods, utilizing the Gibbs energies of formation, were first used to perform calculations on equilibria involving benzene and all 12 of the methylsubstituted benzenes, ranging up to hexamethylbenzene? A method of successive approximations was employed to solve seven independent equations for the seven unknown concentrations, consisting of individual components and groups of di-, tri-, and tetramethyl isomers. Relative amounts within a group of isomers were then calculated from their respective free energies of formation. Since these early investigations, refinements in analytical methods have enabled a very detailed and accurate picture of the composition of complex hydrocarbon mixtures to be obtained. Furthermore, advances in computer technology have allowed systems with a large number of species to be modeled. This has recently been exemplified by the thermodynamic equilibrium calculations on the C-H system by Linton and T ~ r n b u l l . ~These workers performed computer calculations for over 700 equilibrium distributions, with the intention of defining the optimum conditions for processes such as the conversion of coal to liquids. The computer program was able to accommodate up to 100 species simultaneously. Computed amounts were tabulated for compounds that accounted for more than 0.0001% of the carbon in the system. Temperature, pressure, and total hydrogen-carbon atom ratio were (3) Thorne, H. M.; Murphy, W. I. R.; Ball, J. S.; Stanfield, K. E.; Horne, J. W. Ind. Eng. Chem. 1951,43, 20. (4) Hastings, S. H.; Nicholson, D. E. J. Chem. Eng. Data 1961, 6 , 1. (5) Linton, M.; Turnbull, A. G. Fuel 1984, 63, 408.
0 1987 American Chemical Society
Modeling of Oil Shale Pyrolysis
varied, and about 50 of 409 C-H compounds were found to be stable in some range of these conditions. Methane was shown to be the most stable alkane, aromatics were found to increase in stability with ring number, and substituted aromatics were determined to be less stable than parent compounds. The main purpose of the present work is to test the applicability of equilibrium calculations to a range of compounds known to be present in some Australian shale oils, with the overall goal of providing a thermochemical foundation for the pyrolysis of oil shale. The work offers a more detailed compositional analysis than previous studies and includes nitrogen and oxygen species in the distributions. In principle, sulfur could also be considered as a component, and further work is continuing on this topic.
Experimental Section Samples of oil shale were obtained from the Julia Creek region (atomic H/C = 1.2, assay 90 dm3to&) and the Rundle formation (atomic H/C = 1.8, assay 140 dm3 ton-'), both in Queensland, Australia. These two samples represent the extremes of kerogen types, ranging from very aromatic to very aliphatic. Shale oil was produced by retorting -200 mesh material in a Pyrex tube enclosed in a cylindrical furnace programmed from 30 to 600 "C at 5 "C min-'. Inert gas was passed through the tube, and the volatiles were collected in an ice-cooled vapor trap. Bomb pyrolysis experiments were performed by using 1-g samples in sealed and evacuated thick-wall 25-cm3Pyrex vessels. These were heated in a fluidized sand bath a t 500 "C for varying periods of time. Residual carbon on the spent shale was removed by combustion a t 550 "C. Oils were fractionated into acidic, basic, aliphatic, and aromatic materials, by using extraction with NaOH (4 M) and H d 0 4 (20%) and column liquid chromatography on silica Solvent was removed on a spinning-band distillation column, to avoid loss of the light ends. GC analyses were carried out on a 25 m X 0.33 mm ID fused-silica BP-1 (methyl silicone) column programmed from 30 to 300 "C at 4 "C mi&. Dual nitrogen specific and flame ionization detection was employed to measure species eluting from the GC. Relative peak areas within a family of isomers were usually reproducible for successive injections to approximately 10%. GC/MS analysis was accomplished by using a 25 m X 0.2 mm i.d. BP-10 fused-silica column attached to a VG 7035 doublefocusing mass spectrometer. Reconstructed ion summation chromatograms were used to identify most series of isomers. For monoaromatic compounds, the ions m / z 91,105, 106,119, 120, 133, and 134 were selected as most appropriate for the identification of peaks. The ions m/z 141,142,155, and 158 were chosen as most characteristic of the diaromatic species. Monomethylalkanes were identified from their Kovats retention indices and by the m/z ion summation x ( 4 3 + 14n). Equilibrium calculations were carried out with the program CHEMM, which forms part of the CSIRO-NPL THERMODATA system? The method involves a search for the minimum value of the free energy of the system according to the procedure formulated by Eriksson? The total Gibbs free energy of the system containing m compounds is given by
..
i=l
where ni is the amount of moles of substance i, R is the gas constant, n is the total amount in moles in the gas phase, and P is the total pressure. Standard thermodynamicvalues for the majority of compounds of interest in this study were obtained from the experimentally derived data base, consisting of approximately 380 C and H compounds and 120 C, H, N, and 0 compounds, provided with (6)Regtop, R. A.; Crisp, P. T.; Ellis, J. Fuel 1982, 62, 185. (7)Turnbull, A. G.Symp. . . Ser.-Australas. Imt. Min. Metall. 1977, NO.16,49-53. (8) Eriksson, G. Acta. Chem. Scand. 1971, 25, 2651.
Energy & Fuels, Vol. 1, No. 6, 1987 489 the THERMODATA program (JA", NBS,and MRL data). These data are normally quoted to two decimal places, which would imply an accuracy of better than 1%. Unfortunately enthalpy, entropy, and heat capacity values for approximately 30 of the heteroatomic compounds were not available; therefore, these were estimated by using the Benson group-additivity m e t h ~ d . ~ The method of distinguishing between stable and unstable compounds waa essentially the same as that employed by Linton and Turnbull! An ensemble of 50 compounds was initially chosen, then modified by replacing those species that were computed to account for less than 0.0001% total carbon. By inference, if a member of a homologous series was found to be unstable, then higher members were also assumed to be unstable. Approximately 200 combinations of molecules were considered, often at a range of elemental ratios and temperatures.
Results and Discussion Hershkowitz et allo have shown that aliphatic compounds in oil shale aromatize during pyrolysis, and Burnham and Happe'l report that the amount of aromatic carbon in the products, for a range of pyrolysis conditions, can be twice as large as the amount in the raw shale. Wilson et have recently demonstrated that, for Rundle oil shale, pyrolysis at 350-500 O C for 10-240 min gives up to 25% conversion of aliphatic carbon to aromatic carbon. Little data has been reported detailing the composition of the aromatic material formed during controlled oil shale retorting operations. With this in mind, some preliminary studies were first carried out to determine the extent to which compounds unrelated to the starting material could be formed during pyrolysis. Bomb pyrolysis experiments were performed by using saturated normal hydrocarbons, including high density polyethylene and n-pentane. Less than 10% of the polyethylene and n-pentane was converted to solid carbon; however, approximately half of each starting material was converted to gases. Figure 1 illustrates the total ion current response obtained by GC/MS analysis of the n-pentane pyrolysis products. Virtually all the liquid remaining after 1 h at 500 "C is composed of molecules with aromatic nuclei. The majority of these comprise single or diaromatic rings with up to three carbon atoms in side chains. A small amount of polynuclear aromatic (PNA) material is also present. The mechanism of formation of these compounds must involve the condensation of smaller units, and Diels-Alder reactions have been proposed to account for this.13 Similar behavior is observed following pyrolysis of polyethylene at 500 "C for 1h. The GC results presented in Figure 2 illustrate the compounds produced by bomb pyrolysis of polyethylene, and polyethylene mixed with carbon-free Rundle shale ash (1:4 by weight). Also illustrated are the calculated thermodynamic isomer distribution patterns for Cz-and C3-substituted benzenes, and methylnaphthalenes, at 500 "C. Like the pentane pyrolyzate, mono- and diaromatic species are the dominant pyrolysis products. Within any particular group of these isomers, the thermodynamic equilibrium distributions are most closely approximated by products from polyethylene pyrolyzed (9)Benson, S. W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G. R.; ONeal, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Chem. Reu. 1969,69, 219-324. (10)Hershkowitz, F.;Olmstead, W. N.; Rhodes, R. P.; Rose, K. D. In Geochemistry and Chemistry of Oil Shales; Miknis, F. P., McKay, J. F., Eds.; ACS Symposium Series 230;American Chemical Society: Washington, DC, 1983,Chapter 15. (11)Burnham, A. K.; Happe, J. A. Fuel 1984,63, 1353. (12)Wilson, M.A.; Lambert; D. E.; Collin, P. J. Fuel 1985, 64, 1647. (13) Sakai, T.;Nohara, D.; Kunugi, T. Industrial and Laboratory Pyrolyses; Albright, L. F., Crynes, B. L., Eds.; ACS Symposium Series 32;American Chemical Society: Washington, DC, 1976; Chapter 10.
Charlesworth
490 Energy & Fuels, Vol. 1, No. 6, 1987 16!8!
~'~#?!T':oo
?'i8n
XI
t
S C A N N O / R E T E N T I O N TIME [ M I N I
X8
POLYNUCLEAR AROMATICS
-t I
30
50
THERMODYNAMIC DIST.
T OC
100
Figure 2. GC analysis of liquid products from pyrolysis of high-density polyethylene for 1h at 500 O C : top, polyethylene; middle, polyethylene mixed with carbon-free shale ash (1:4 by
S C A N N O / R E T E N T I O N TIME [ M I N I
Figure 1. GC/MS analysis of liquid products from pyrolysis of n-pentane for 1h at 500 O C : top, mono- and diaromatica; bottom, polynuclear aromatics.
in the presence of shale ash. These data support the concept that equilibrium is attained, at least on a limited scale, during pyrolysis of simple aliphatic hydrocarbons and that thermodynamic calculations may assist in predicting the behavior in more complex systems. It is also clear that the mineral matter in Rundle shale ash has catalytic properties that enable high-temperature cracking reactions to come closer to equilibrium. Clay minerals, particularly the expandable types, are well-known for their capacity to promote isomerization processes, largely due to their ability to facilitate carbonium ion rearrangements at acidic sites.14 The mineral matter in Rundle oil shale includes a large percentage of the clay mineral m~ntmorillonite,'~ which is certainly one of the most catalytically active of the clays.16 This most probably accounts for the tendency of the shale ash to modify the product distribution relative to the mineral free systems. Figure 3 shows the total ion current, the s u m of selected ions, and peak identifications derived from the GC/MS analysis of the aromatic fraction in Rundle shale oil. The immediately obvious feature of these data is their close similarity to the isomer distribution patterns produced by the bomb pyrolysis experiments with aliphatic model compounds. The data listed in Tables I and I1 are quantitative estimates of the proportions of a variety of com~~
~
(14) Pines, H. The Chemistry of Catalytic Hydrocarbon Conversions
Academic: New York, 1981. (15) Charleaworth,J. M. Fuel 1986, 65, 1159. (16) Galway, A. K. Nature (London) 1969,223,1257.
1
weight); bottom, distribution of selected isomer groups predicted from thermodynamic calculations. Table I. Equilibria Attained after Oil Shale Pyrolysis at 5 OC min-' thermodynamic equilibria, mol % Rundle Julia compd 400 O C 600 "C oil Creek oil C8Benzenes 7.8 12.8 19 ethyl 25 21.7 p-xylene 54 37 20.1 } 48.4 44.6 m-xylene 22.2 23.2 39 28 o-xylene
CBBenzenes isopropyl n-propyl 1-methyl-3-ethyl 1-methyl-4-ethyl 1-methyl-2-ethyl 1,3,5-trimethyl 1,2,4-trimethyl 1,2,3-trimethyl
0.9 1.8 15.1 9.8 5.4 17.2 44.4 5.4
1.6 3.8 20.8 12.5 8.9
2-methyl 1-methyl
CI1Naphthalenes 60.3 58.9 41.1 39.7
12.1
35.2 5.1
2 8 16 10 6 18 30 11
0 4 20
54 46
60 40
10 15 9 34 8
pounds within individual aromatic and heteroaromatic groups of isomers found in the two types of shale oil. A fair to good correspondence exists between the predicted and experimentally observed distributions within most isomer series. The agreement is best for the Julia Creek shale oil, which is derived from the very aromatic type of kerogen. A number of obvious disparities exist for the Rundle shale oil; for instance, the sum of the m- and p xylenes is much less than expected and the 3- and 4methylpyridine isomers predominate in the reverse order
Modeling of Oil Shale Pyrolysis
Energy & Fuels, Vol. I , No. 6, 1987 491
Table 11. Equilibria Attained after Oil Shale Pyrolysis at 5 "C min-' thermodynamic equilibria, mol % Rundle Julia compd 400 "C 600 "C oil Creek oil C7 Phenols m-cresol 57.4 52.0 42 a o-cresol 37.2 41.5 44 a p-cresol 5.5 6.5 14 a 2-methyl 3-methyl 4-methyl
58.7 16.0 25.3
2-ethyl 3-ethyl 4-ethyl 2,3-dimethyl 3,4-dimethyl 2,4-dimethyl 2,5-dimethyl 2,6-dimethyl 3,5-dimethyl
1.0 0.6 1.0 2.6 5.2 47.6 15.5 23.9 2.5
C6 Pyridines 55.4 20.1 24.5
C7 Pyridines 2.1 1.8 2.1 3.6 7.1 41.4 17.4 } 20.8 3.7
47 33 20
63 12 25
8 0
8 0 0
9 7 33
9 16 22
34 2
40 4
7
Not determined.
to that anticipated. Overall the relative proportions are too close to dismiss the data as coincidental. A number of groups of isomers of aliphatic compounds from shale oil were also investigated for any indication of a correspondence between experimental and thermodynamic distribution patterns. Figure 4 presents the results of GC/MS analysis of the saturated fraction from Rundle shale oil, with selected ion summation used to identify the monomethylalkanes. The calculated equilibrium distribution at 500 "C for the four isomers of methylnonane is shown in Figure 4 for comparison. Although these compounds may only represent a small percentage of the total variety of isoalkanes that can occur, the monobranched species are usually found in larger proportions than the more highly branched isomers." These compounds have considerable practical significance in terms of the performance characteristics of fuels because, for automotive gasoline, branching influences the octane rating and, for middle distillate fuels, the freeze point and cetane number are determined to a large extent by branching. Weitkamp18 has shown that in the C9 to CI5 range, a single methyl branch in the 3-7-positions causes a melting point depression of around 40-70 "C,while branching in the 2-position causes a depression of about 20-45 "C. The experimental data suggests that for Rundle shale oil the 2-methyl isomers are the most predominant in each group, whereas calculated equilibrium concentrations for the 3and 4-methyl isomers at 500 "C are greater than that for the 2-isomer. At low levels of conversion the isomerization of n-alkanes by carbonium ion rearrangements depends to a great degree on the relative rates of the various steps inv~lved.'~ A preference for the higher isomers is usually observed, Le., 3 < 4 < 5 etc., and because the branching mechanism involves a protonated cyclopropane intermediate, the 2methyl isomers are formed at about half the rate.l* Catalytic cracking on the other hand leads to a dominance of &scission reactions, which produce the 2-methyl isomer as the dominant product14(although there are exceptions to this18). It is highly probable that these kinetic factors (17) Rossini, F. D. J. Chem. Educ. 1960, 37, 554. (18) Weitkamp, J. Ind. Eng. Chem. Prod. Res. Deu. 1982, 21, 550. (19) Schulz, H. F.; Weitkamp, J. H. Ind. Eng. Chem. Prod. Res. Deu. 1972, 11, 46.
S C A N N O . / R E T E N T I O N TIME (MINI
Figure 3. GC/MS analysis of the aromatic fraction of Rundle shale oil: (a) total ion current; (b) selected ion summation for monoaromatics; (c) selected ion summation for diaromatics.
are important during the cracking of the saturated units in kerogen, and this subsequently determines the ultimate distribution pattern for the monomethylalkanes. A further example, which illustrates clearly the influence of kinetic factors on the product distribution, is provided by the isomer distribution pattern of the n-alkenes. Figure 5 presents the results of a GC analysis of the aliphatic fraction from Rundle oil shale, which has been produced by retorting under programmed heating conditions, starting with a mixture of oil shale and carbon-free shale ash (1:l by weight). These data should be compared to the chromatogram in Figure 6 showing the composition of the oil evolved from a retorting experiment in which no carbon-free shale ash was mixed with the starting material. The most obvious difference between these two oils is the appearance of the internal isomers of the n-alkenes in the products from the oil shale/shale ash mixture. Previous studies have shown that 1-alkenes are the major alkene isomer formed by the thermal cracking of pure n-alkanes.20 The present data confirm that isomerization of alkenes is (20) Fabuss, B. M.; Smith, J. 0.; Lait, R. I.; Borsanyi, A. S.; Satterfield, C . N. Ind. Eng. Chem. Process Des. Deu. 1962, 1 , 293.
492 Energy & Fuels, Vol. 1, No. 6, 1987
Charlesworth
__
RUNOLE+ 2: u = u 3 5 , 1 7 1 + 8 5 + 9 9
c9 i
I
D I S T R I B U T I O N 5OO0C
c11
ClO
THERMODYNAMIC s
4- 3-methyl 2- nonane
1 id00 15: 37
SCAN NO./RETENTION TIME (MINI Figure 4. GC/MS analysis of the saturated fraction from Rundle shale oil, with selected ion summation to identify monomethylalkanes. The thermodynamic equilibrium distribution pattern of the methylnonane isomers is also shown. a I-ALKENE b 4-ALKENE c 3-ALKENE dhe c i s a t r a n 8 2-ALKENE
c10
Table 111. Equilibrium Distribution with Graphite, Ammonia, and Water Included ( P = 1 atm; C:H:N:O = 1:1.75:0.0250.025) wt%
comud hydrogen graphite
400 "C 2.877 57.653
methane ethane
33.407
CZH6
NH3 H20
ammonia water
formula H2
C CH, ,
;
200
100
30
TEMPER A TU R E % , Figure 5. GC analysis of the saturated fraction from Rundle shale oil, produced by retorting a mixture of oil shale and carbon-free shale ash (1:l by weight).
6
30
"-ALKANE
1-ALKENE
2w
1M)
T 'c
Figure 6. GC analysis of the whole oil from Rundle oil shale.
promoted by the shale ash minerals, and the internal alkenes that are produced are indeed those which are the most thermodynamically favored.' The absence of significant levels of internal alkenes in oil evolved under standard retorting conditions is somewhat surprising in view of the fact that this type of isomerization should be a rapid process on aluminosilicate^,^^ which are known to be the major mineral component of the oil shale.15 This suggests that whenever short contact time retorting is involved, the minerals do not play an important role in modifying the product composition. A more extensive test of the thermodynamic equilibrium model involves an examination of the relative ratios of the sum of particular groups of isomers, for example, the sum
0.003 2.932 3.100
600 "C 9.429 77.151 7.343 0.000 2.930 3.100
of isomers in each group of alkylbenzenes. Of necessity this requires that the thermodynamically probable distribution be calculated by using a complete set of the most stable compounds spanning the isomer groups of interest. Table I11 lists the results of a sample preliminary computation in which graphite, ammonia, and water were included among 50 stable species, selected on the basis of the results presented by Linton and T ~ r n b u l l .It~ is obvious that almost all the carbon in the system occurs as methane or graphite, while ammonia and water account for all the oxygen and nitrogen. This does not reflect the known composition of the products from any conventional oil shale pyrolysis experiment. It therefore seems justifiable to exclude these compounds from model system calculations on the grounds that kinetic barriers prevent their formation in large quantities. Table IV presents the distribution of the most stable compounds for a more realistic model calculation in which very stable and dominant products such as graphite, ammonia, and water have been rejected. These values should be compared with the results in Table V, which relate to the same system except that PNA's with three or more fused rings are excluded. The calculated equilibrium composition clearly depends to a large extent upon the incorporation of PNA's in the computations. In particular, the ratios of aromatic isomer groups are very dependent on these molecules. This is quantitatively highlighted by the data in Table VI. For instance, the ratio of pyridine to quinoline varies by a factor of 40 depending on whether
Modeling of Oil Shale Pyrolysis
Energy & Fuels, Vol. 1, No. 6,1987 493
Table IV. Equilibrium Distribution with Graphite, Ammonia, and Water Excluded (P= 1 atm; C:HN:O = 1:1.75:0.025:0.025) wt%
formula H2 CH, C2H6
C3H60 C4H6N C6H6N CBH6 CGH6O C6H7N C7H80 CBH7N ClOH8 C16H10
compd hydrogen methane ethane acetone pyrrole pyridine benzene phenol methylpyridines cresols quinoline naphthalene pyrene
400 "C 0.047 31.721 0.016 8.610 1.975 9.346 0.004 2.156 1.268 0.100 1.400 0.001 43.324
600 "C 0.511 29.925 0.024 6.200 2.042 9.704 0.033 5.999 0.626 0.168 1.578 0.007 43.145
Table VI. Calculated and Experimental Ratios of Isomer Groups Found in Rundle Shale Oil tol:C8Ph:CSPh:Nap: Me-Na.p:C,-NaD py:Me-py:quin PNA" present 1.00:0.07:0.16 0.19:0.00:0.00:1.00:0.39:0.00 PNA" absent 0.16:0.02:1.00 0.01:0.00:0.00:1.000.07:0.00 0.30:1.00:0.34 0.57:0.82:1.00:0.29:0.33:0.28 exptl
" PNA = polynuclear aromatics (pyrene, phenanthrene, and anthracene). Table VII. Thermodynamic Stability of Compound Classes elements stable comDounds unstable comDounds
Table V. Equilibrium Distribution of a Selection of Most Stable Compounds at 1 atm Total Pressure for the C, H,N, and 0 System with a n Atomic Ratio of 1:1.75:0.025:0.025 wt %"
no. formula
compd
400 OC 600 "C 0.024 0.299
hydrogen graphite 31.091 methane 0.000 ethylene 0.030 ethane 0.000 propene 0.000 propane 1.024 acetone 0.073 pyrrole 0.000 pyrrolidine 1.290 pyridine 1.317 benzene 13.501 phenol 0.198 2-methylpyridine 0.054 3-methylpyridine 0.085 4-methylpyridine 0.145 toluene 0.685 m-cresol 0.444 o-cresol 0.065 p-cresol 0.000 styrene 0.000 ethylbenzene 0.003 m-xy 1ene 0.001 o-xylene 0.001 p-xylene 19.518 quin o1ine 0.000 indene 0.000 m-ethyltoluene 28 27.105 naphthalene 29 1.245 I-methylnaphthalene 30 1.893 2-methylnaphthalene 31 0.074 biphenyl 32 0.004 I-ethylnaphthalene 33 0.006 2-ethylnaphthalene 34 0.010 1,2-dimethylnaphthalene 35 0.019 1,3-dimethylnaphthalene 36 0.006 1,4-dimethylnaphthalene 37 0.007 1,5-dimethylnaphthalene 38 0.017 1,6-dimethylnaphthalene 39 0.019 1,7-dimethylnaphthalene 40 0.016 2,3-dimethylnaphthalene 41 0.016 2,6-dimethylnaphthalene 42 0.016 2,7-dimethylnaphthalene 43 2-ethyl-3-methylnaphthalene 0.000 44 phenanthrene 45 anthracene 46 47 Pyrene Compounds 1 and 3-44 are present simultaneously. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
29.556 0.018 0.040 0.001 0.000 0.661 0.202 0.000 2.720 3.624 14.527 0.165 0.059 0.073 0.230 0.357 0.286 0.045 0.003 0.001 0.002 0.001 0.001
16.985 0.004 0.000 28.103 0.759 1.089 0.134 0.002 0.003 0.004 0.006 0.002 0.002 0.006 0.006 0.005 0.005 0.005 0.000
pyrene, phenanthrene, or anthracene are included. Table VI1 qualitatively summarizes the results of calculations encompassing all the major classes of compounds
? C 1 0 AROMATICS
II I C,H,O COMPOUNDS
300
400
500
600
700
TEMPER A TUR E ,%
F i g u r e 7. Thermodynamic equilibrium distribution of carbon and molecular hydrogen, as a function of temperature, a t a C: H:N:O ratio of 1:1.75:0.01:0.025.
for which the necessary thermodynamic data is available that could be considered to be relevant to a study of shale oil thermochemistry. Aromatic and heteroaromatic molecules become increasingly stable as the ?r-bonding ring system extends. Addition of a single carbon atom substituent to a n-bonding ring reduces the stability relative to the parent compound by an order of magnitude. Saturated compounds with chains longer than three carbon atoms have no significant stability under these conditions. Clearly, the entire product from the pyrolysis of oil shale cannot be in thermodynamic equilibriumbecause shale oils are composed, to a large extent, of alkanes and alkenes with long chains (Figures 5 and 6). These compounds would be expected to be unstable at high temperatures. It must be concluded that the kinetics of the cracking reactions that propagate the degradation of these molecules are slower than the rate at which the species escape from the hot zone in the furnace. Figures 7 and 8 show examples of the distribution of carbon and hydrogen among the major stable products at two extreme ratios of atomic carbon to nitrogen. Little variation in the relative amounts of each compound class occurs from 300 to 750 "C, and increasing the ratio of
Charlesworth
494 Energy & Fuels, Vol. 1, NO.6, 1987 C / O = l O O , P = l ATM
C / N = 1 0 , P=lATM
CRESOLS
/
" o 1 Y C I O AROMATICS
80
c
i
It
C,H.N COMPOUNDS
ACETONE
TEMPERATURE,%
Figure 8. Thermodynamic equilibrium distribution of carbon and molecular hydrogen, as a function of temperature, at a C:
H:N:O ratio of 1:1.75:0.1:0.025. C i N = 1 0 0 , P:l
I
r/
I
OUlNOLlNE
PYRIDINE
l
I
(1
0
1I
20
PYRROLE
300
400
500
400
600
500 TEMP E R A T uR E
700
PC
Figure 10. Thermodynamic equilibrium distribution of oxygen, as a function of temperature, at a C:H:N:O ratio of 1:1.75:0.025:0.01.
ATM
METHYLPYRIDINES
w
300
600
700
TEMP E R A T uR E ,'c
Figure 9. Thermodynamic equilibrium distribution of nitrogen, as a function of temperature, at a C:H:N:O ratio of
1:1.75:0.01:0.025.
nitrogen (or oxygen) to carbon simply results in a net increase in the relative amount of carbon in heterocompounds. Molecular hydrogen becomes a major stable component above 600 "C, while methane tends to decline in proportion at these higher temperatures. Figures 9 and 10 show the distribution of nitrogen and oxygen in the most stable of the heterocompoundsthat this study has encompassed. Almost no change occurs in the relative proportion of the major stable nitrogen species; however, the oxygenated products display a more pronounced trend, with phenol becoming more dominant at the higher temperatures. The ratio results presented in Table VI reveal a marked discrepancy between the experimental and calculated behavior at both extremes of the computations (with PNA's present or absent). By inference, it can be concluded that unrestricted reactions between isomer groups, including such processes as disproportionation and transalkylation, do not influence the distributions produced by short contact time retorting. However, it is conceivable that if the calculations were modified to allow for only a
Table VIII. Isomer Distribution (mol W )following Equilibration of Shale Oil over Shale Ash at 500 "C original after after Rundle 15 35 thermodynamic comud shale oil min min eauilibria C8 Benzenes ethyl 25 28 29 10 p-xylene 37 38 43 67 m-xylene } o-xylene 39 34 28 23 isopropyl n-propyl 1-methyl-3-ethyl 1-methyl-4-ethyl 1-methyl-2-ethyl 1,3,5-trimethyl
1,2,4-trimethyl 1,2,3-trimethyl
Cg Benzenes 2 1 8 8 16 19 10 10 6 3 18 17 30 27 11 15
0.8 0.1
0.9 0.7
1 6 29 10 5 18 21 11
1 11 7 17 39 5
1.2 15.6
126 1.5 x
3 18
1013
limited degree of equilibrium, for example, restricting the computations to specific reactions such as benzene + xylene e 2 toluene toluene + trimethylbenzene s 2 xylene it may be possible to develop a reaction network that would predict with better precision the relative ratios of the sum of isomers. This is beyond the scope of the present work. The effects of longer residence times on the degree to which chemical equilibrium is attained are highlighted by the results of bomb pyrolysis measurements, illustrated in Figures 11 and 12 and summarized in Table VIII. These relate to shale oil mixed with carbon-free shale ash (1:4 by weight), maintained at 500 "C for up to 35 min. The rapid disappearance of the n-alkanes and 1-alkenes in the hydrocarbon fraction of the oil (FID trace) confirms the predictions of the thermodynamic model as discussed above. In particular, after 35 min at 500 "C, the vast majority of the saturated hydrocarbons above C8 have disappeared leaving behind, and f or forming, aromatic compounds with molecular weights ranging up to that for C,-substituted naphthalene. The data also indicate that
Modeling of Oil Shale Pyrolysis
Energy & Fuels, Vol. 1, No. 6,1987 495
Q+
1-40
PYRIDINES
, QUINOLINES
500°C, 15 MIN OVER ASH.
TEMPERATURE ,
I 30
100
150
Z b
-c do
330
Figure 13. GC analysis, with nitrogen specific detection (TSD), of Rundle shale oil. Peaks 1-40 are identified in ref 21.
io
200
100
T "C
Figure 11. GC analysis of liquid products from bomb pyrolysis of Rundle shale oil mixed with carbon-free shale ash (1:4 by
weight).
n-4
*
il
I
~I ~
5 0 0 ° C , 15 MIN OVER ASH
0 v)
A'
/ I
40
500°C ,35 MIN OVER A S H
' I
100
200 TC ',
Figure 12. GC analysis, with nitrogen specific detection (TSD), of liquid products from bomb pyrolysis of Rundle shale oil mixed with carbon-free shale ash (1:4 by weight).
not a great deal of change occurs in the relative proportions of the individual isomers and also in the relative proportions of each isomer group.
A similar situation appears to hold for the organic nitrogen compounds. Figure 13 shows the nitrogen specific detection GC analysis of a sample of freshly retorted Rundle shale oil. The majority (50-60%) of nitrogen is distributed among n-alkanenitriles ranging up to 35 carbon atoms in length. The other nitrogen-containing molecules are largely basic species comprised of substituted pyridines and quinolines. Peaks numbered from 1 to 40 have previously been identified by GC/MS.21 After 15 min at 500 "C, all the larger n-alkanenitriles (XI,) no longer appear in the liquid product, and after 35 min almost all the liquid remaining is composed of substituted pyridines and quinolines. The smaller amount of pyridine in the pyrolysis oil from these longer contact time experiments is likely to be due to adsorption onto the clay component of the mineral matter.22 In summary, from the data available, it appears that aromatics with molecular weights at least up to that of pyrene are formed to some extent during pyrolysis. Within a given isomer group, species are approximately in the thermodynamically probable ratios, although a number of instances exist where this is not true. Calculations based on an unrestricted large-scale equilibrium model have so far been unable to accurately predict the relative ratios of several of the major stable isomer groups. It appears that the mineral matter in Rundle oil shale has a catalytic effect on isomerization reactions when it is present in large amounts as a carbon-free material or, alternatively, on isomerization reactions during long contact time experiments. Bomb pyrolysis of oil shale results in a progression toward equilibrium. In previous work related to the thermodynamic distribution pattern of isomers from a low-temperature coal tar p y r o l y ~ a t eno , ~ ~correspondence between most probable distributions of isomers and experimental results was found, and it was therefore concluded that the products were to a significant degree determined by the structure af the original coal molecule. It must be pointed out that in the present study the fact that the distribution pattern within several groups of isomers is close to the most probable distribution may well indicate that the biological and geochemical processes that formed the kerogen have favored the production of substitution patterns with the minimum free energy. Acknowledgment. The author thanks P. J. Sanders and V. T. Borrett for their assistance. (21) Charlesworth, J. M. Fuel 1986, 65, 979. (22) Charlesworth, J. M. Ceochim. Cosmochim. Acta 1986,50, 1431. (23) Karr, C., Jr. J . Phys. Chem. 1960, 64, 462.
496
Energy & Fuels 1987, 1, 496-501
Registry No. 1,1333-74-0; 2,7782-42-5; 3,7442-8; 4,74-85-1; 5, 74-84-0; 6, 115-07-1; 7, 74-98-6; 8, 67-64-1; 9, 109-97-7; 10, 123-75-1; 11, 110-86-1; 12, 71-43-2; 13, 108-95-2; 14,109-06-8; 15, 108-99-6; 16, 108-89-4; 17, 108-88-3; 18, 108-39-4; 19,95-48-7; 20, 106-44-5; 21,100-42-5; 22,100-41-4; 23,106-42-3; 24,108-38-3; 25, 95-47-6; 26, 91-22-5; 27, 95-13-6; 28, 620-14-4; 29, 91-20-3; 30, 90-12-0; 31,91-57-6; 32, 92-52-4; 33, 1127-76-0; 34,939-27-5; 35, 573-98-8; 36,575-41-7; 37,571-58-4; 38,571-61-9; 39,575-43-9; 40, 575-37-1; 41,581-40-8; 42, 581-42-0; 43, 582-16-1; 44, 31032-94-7;
45,85-01-8; 46,120-12-7; 47,129-00-0; CsHS(CH2)ZCH3,103-65-1; CBHSCH(CHS)Z, 98-82-8; NHB, 7664-41-7; HzO, 7732-18-5; 1methyl-4-ethylbenzene, 622-96-8; l-methyl-2-ethylbenzene, 61114-3; 1,3,5-trimethylbenzene, 108-67-8; 1,2,4-trimethylbenzene, 95-63-6; 1,2,3-trimethylbenzene, 526-73-8; 2-ethylpyridine, 10071-0; 3-ethylpyridine, 536-78-7; 4-ethylpyridine, 536-75-4; 2,3dimethylpyridine, 583-61-9; 3,4-dimethylpyridine, 583-58-4; 2,4dimethylpyridine, 108-47-4; 2,5-dimethylpyridine, 589-93-5; 2,6dimethylpyridine, 108-48-5; 3,5-dimethylpyridine, 591-22-0.
Conversion of Hydrocarbon Gases to Liquid in the Fluidized-Bed Retort? Thomas T. Coburn,* Michael W. Droege, and Richard G. Mallon Lawrence Livermore National Laboratory, Livermore, California 94550 Received June 22, 1987. Revised Manuscript Received August 24, 1987
A free-radical steady-state mechanism has been developed to explain qualitative and quantitative aspects of the generation of hydrocarbon gases in the solid-recycle fluidized-bed retort. The mechanism relies on what is known about butane cracking. The 1- and 2-butyl free radicals have been studied as representative intermediates responsible for the light gases methane, ethane, ethylene, and propylene. The ratio of these gases formed from oil shale retorting exactly matches the ratio obtained from pyrolysis of n-butane and supports a butane-like cracking mechanism. In a variety of fluidized-bed retorting experiments conducted with Lawrence Livermore National Laboratory’s (LLNL) 2 ton/day retort, free-radical precursors of the hydrocarbon gases come to a steady state that efficiently equilibrates intermediates. The steady state is established rapidly without need for a gas recycle. Analogous reactions are possible for the entire homologous series of free radicals; as a result, the retort operator has considerable flexibility in choosing the nature of hydrocarbon products. A steady-state mechanism, in which alkenes convert precursors of gaseous hydrocarbons to liquid product, offers an insight that can be used to obtain higher oil yields.
Introduction In our initial report,l we developed a working hypothesis to explain the formation and distribution of hydrocarbon gases produced in a fluidized-bed retort. The hypothesis grew out of our attempts to identify specific local equilibria and understand the impact, if any, of chemical equilibria on product distribution from an oil shale retort. Kinetic control defines retort products. At total equilibrium, the significant products are hydrogen, carbon, and methane; therefore, in a well-designed oil shale retorting experiment, equilibrium is avoided by rapid removal of pyrolysis products. Local equilibria involving a limited This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California and shall not be used for advertising or product endorsement purposes.
number of metastable compounds may still be established even when the system is largely under kinetic control; however, to achieve local equilibria, temperatures somewhat higher than the retort operating temperture are generally requireda2b Reactive intermediates, such as hydrocarbon free radicals, equilibrate rapidly compared with the compounds that are isolated as final products. We report here our efforts to identify local equilibria involving reactive intermediates that form in the solid-recycle fluidized-bed retort. We specify a set of reversible reactions (i.e., equilibria) that make up a steady state that defines the gas-phase hydrocarbon chemistry. Our preliminary tests of usual methods for shifting equlibria show that these techniques can be used to obtain an improved product distribution. In particular, the chemistry associated with the free-radical steady state suggests that an olefin-rich sweep gas will increase oil formation at the expense of gas production. (1) Cobum, T. T.; Droege, M. W. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1987.32I1). 127-32. ~~,~ (2)B & , A. K.; Ward, R. L. In Oil Shale, Tar Sands, and Rehted Materials; Stauffer, H. C., Ed.; ACS Symposium Series 163; American Chemical Society: Washington, DC, 1981;pp 79-92. (3) Coburn, T. T.; Crawford, R. W.; Gregg, H. R.; Oh, M. S. Proceedings, I986 Eastern Oil Shale Symposium; Kentucky Energy Cabinet Laboratory: Lexington, KY, 1986;pp 291-302. ~~
0887-0624/87/2501-0496$01.50/00 1987 American Chemical Society
