Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 1117-1125
rately, the binodal curves were not. Since the operating region falls in the section where the binodal c w e s become open, the unsatisfactory simulation results. Thus, trying to match the binodal curve at the section most sensitive to simulation gives an idea of the allowable prediction errors. Figure 1 also shows the binodal curves mentioned above. From Table 111, and Figures 2 and 3 also, the sensitivity of simulation to the phase equilibria prediction errors can be seen. When the VLE parameters are used in place of the LLE parameters, although the LLE prediction are acceptable (Table III), they are not accurate enough in terms of the required accuracy of simulation (Table 11).
Conclusions The satisfaction of the simulation/design requirements for the different examples studied points to the reliability and the wide range of applicability of the proposed procedure. Numerical results also show the unreliability of procedures employing VLE parameters for prediction of LLE and vice versa. Although, the proposed procedure needs some form of phase equilibrium data for the corresponding process, the actual amount of data required are quite small. In some cases, only data relating the critical components or some limited amount of plant data are enough. Preliminary sensitivity analysis points to the possible existence of high sensitivity of prediction errors to the simulation results. Current work is extending the sensitivity studies and developing a special purpose table of adjusted group interaction parameters for simulation and design of industrial separation processes. Nomenclature = liquid-phase fugacity of component i
fy
1117
= vapor-phase fugacity of component i
Ki= equilibrium ratio for VLE of component i
p ; = calculated property of component of set j used for
verification of adjusted/estimated parameters p $ = desired property of component set j to be matched by
adjusting parameters
wi = weighting factor defined in eq 9 x i = liquid-phase composition of component i yi = vapor-phase composition of component i Greek Letters
p. - distribution ratio for LLE of component i C $ \ i liquid-phase fugacity coefficient of component i C$i = vapor-phase fugacity coefficient of component i yi = activity coefficient of component i Literature Cited Caminos, A. A.; Gani, R.; Brignole, E. A. Comput. Chem. Eng. 1984, 8,127. Christiansen, L. J.; Michelsen, M. L.; Fredensiund, Aa. Paper presented at the 12th Symposlum on Computer Applications in Chemical Enplneerinp, Mon.. treaux; 1980. DeFre, R. M.; Verhowye, L. A. J. Appl. Chem. Biotechnol. 1976, 26 (9), 469.
Devandran, M.; Bhaskara Rao, B. K. Hydrocarbon Process. 1978, 5 5 , (ll),
237. Fredenslund, Aa.; Gmehllng, J.; Rasmussen, P. "Vapor-Liquid Equilibria using UNIFAC"; Elsevier: Amsterdam, 1977. Gmehling, J.; Rasmussen, P.; Fredensiund, Aa. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 118. Hernandez, M. R.; Gani, R.; Romagnoii, J. A,; Brignole, E. A. Proceedings of Foundations of Computer Aided Process Design, Snowmass, CO, 1983. Magnussen. T.; Rasmussen, P.; Fredenslund, Aa. Ind. Eng . Chem. Process Des. Dev. 1981, 20, 331. Mukhopadhyay, M.; Dongaonkar, K. R. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 521. Naphtali, L. M.; Sandhoim. D. P. AIChEJ. 1971, 77, 148. Ssrensen, J. M.; Ark, W. "Liquid-Liquid Equilibrium Data Collection: Ternary and Quaternary Systems"; Chemistry Data DECHEMA: Frankfurt Maine, 1980;Chemistry Data series, Voi. V, Part 3. Roche, E. C.; Br. Chem. Eng. 1969, 14 (lo),1393.
Received for review March 8, 1984 Revised manuscript received October 31, 1984 Accepted December 7, 1984
Oil Shale Pyrolysis. 1. Time and Temperature Dependence of Product Composition John M. Charlesworth Materials Research Laboratories, P.0. Box 50, Ascot Vale Victoria, 3032 Australia
Continuously operating horizontal oil shale retorts offer the potential for selective fractional collection of pyrolysis products. The predominant species in the vapors at short contact times or low temperatures are aromatics, isoprenoids, and saturated cyclic and branched compounds. However, in the middle and later stages of the reaction the major components in the vapor are 1-alkenes and n-alkanes. The magnitude of the compositional variation with conversion appears to be related to the amount of 0, N, and S In the kerogen. Infrared absorbance results show that the destruction of a significant number of the functional groups containing heteroatoms occurs very early in the pyrolysis. The l-alkene to n-alkane ratios are dependent on the pyrolysis temperature, the type of oil shale, and to a lesser extent the degree of conversion. A reexamination of the temperature dependency of the 1-alkene to n-alkane ratios obtained previously for the pyrolysis of n-hexadecane shows that the theoretical relative reaction rates of alkyl radicals is approximately that observed experimentally.
Introduction During the pyrolysis of oil shale the insoluble organic matter is converted to a useful, although commercially unproven, substitute for natural crude oil. From a practical viewpoint, the variation in the composition of the oil vapors evolved from the rock, as a function of both time and temperature, is important because of the potential for 0196-4305/85/1124-1117$01.50/0
selective fractional collection of the products during the retorting operation. This is particularly relevant in the case of the Superior Circular Grate process (Weichman and Knight, 1979) and the Allis-Chalmers process (Faulkner and Weinecke, 1983) in which heating occurs in a number of horizontal zones along the length of the retort. Collectors placed at appropriate intervals enable the vapors 0 1985 American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
/
/////////////
V I
Figure 1. Flow diagram illustrating the main features of the modified gas chromatograph.
to be trapped in the order of their evolution during the continuous operation of the retort. Little information has been published describing the likely time dependence of the oil composition, and it has sometimes been assumed that the rate of evolution will simply reflect the volatility of the pyrolysis products (e.g., (Weichman and Knight, 1979). Data illustrating any differences in the time dependence of the product composition from shales of different geochemical classifications are also scarce. The major aim of the present work is therefore to establish if a worthwhile fractionation of the oil can be carried out during the retorting operation and to interpret the composition of the products in terms of the retorting parameters and the chemical composition of the kerogen. Methods for monitoring oil shale retorting processes fall into two categories. The first class encompasses those techniques that focus on the changes in properties of the oil shale itself. Most common is thermogravimetric analysis (TGA), in which the weight of the shale is monitored, usually as a function of temperature (Earnest, 1982). This has one major disadvantage in that not all the measured weight loss is due to the liberation of organic molecules. In particular, dehydration and decomposition reactions of the minerals can yield appreciable quantities of water and carbon oxides. This problem can be overcome by examining the evolved products from the TGA unit, or from any other suitable pyrolysis apparatus, by using appropriate collection and detection devices. As the hydrocarbons are the main products of interest in the present study, the conventional flame ionization detector (FID) employed in most gas chromatographs, was chosen as ideally suited to this task. Passing the pyrolysis products directly to the FID enables the kinetics to be followed (Wallman et al., 1981), and interposing a gas sampling valve and chromatographic column between the retort and the FID allows the vapor composition to be monitored. Experimental Section Apparatus. Figure 1shows the important features of the modified gas chromatograph used in this work. The retort consisted of a cylindrical stainless steel tube (10 X 0.5 cm) enclosed in a temperature-controlled aluminum block operating in the range 25-600 "C and programmable at rates up to 40 deg. min-'. A maximum of 25 mg of powdered material was introduced into the retort using a cylindrical stainless steel piston (0.25 cm i.d.) hollowed
out in the central portion to accomodate the sample. After insertion of the sample, the piston was immediately rotated and the shale deposited onto the retort inner surface. The sealing arrangement at the entrance to the retort consisted of a Swagelok nut and graphite ferrule making a gas tight seal around the piston circumference. In at least one type of pyrolysis unit previously employed in studies of kerogen decomposition (van de Meent et al., 1980a), appreciable condensation of products within the apparatus has been acknowledged; however the design of the present system avoids cool zones and ensures the maximum removal of volatile products. In normal operation 0.2 to 2 mg of accurately weighed sample crushed to -53 pm was pyrolyzed and the products were swept into the GC by using preheated inert carrier gas at a flow rate of 20 cm3 min-'. In the first mode of operation the products were swept directly to the FID with the GC oven detector, and injection block maintained at 400 "C. In the analytical mode of operation the pyrolysis products were directed through a high-temperature Valco six-port gas sampling valve equipped with a 0.7-cm3 sampling loop. The valve, entrance, and exit gas lines were maintained at 300 "C and insulated from the rest of the apparatus in a separate enclosure. At the chosen reaction time the fraction of vapor filling the loop was redirected through the chromatography column. In most cases a vitreous silica bonded phase WCOT column was employed (SE 30,25 m X 0.33 mm, Ne, = 90 000). The oven was temperature programmed from 30 to 270 "C at 4 O C min-', and He carrier gas at a flow rate of 1 cm3 min-' was used. Peak areas were measured by using a Spectra-Physics 4270 integrator. Data Handling. Unlike several of the other common GC detectors, which respond to the concentration of material in the carrier gas, the FID response factor, R, is proportional to the mass flow rate dmldt, R = K1 dm/dt
(1)
where K1 is a calibration constant. A t time tl after the commencement of the pyrolysis, the mass of product, m,,, which has passed through the detector is given by 1 m,, = dt K1 0
-5"R
The fractional conversion, a,of the kerogen is given by cy
= Jt'R
d t / S 0m R dt
(3)
A large number of relative FID response factors have been determined by Dietz (1967) for a selection of organic and inorganic compounds. From these data K , for the hydrocarbons that are commonly found in oil shale pyrolysis vapors appear to vary by around 15% from the average value, while the nonorganics (e.g., H2, COz, CO) are not detected. The heteroatomic compounds are intermediate in response, but since the major proportion of the oil is comprised of alkenes, alkanes, and aromatics no serious error is expected to arise from this effect. At temperatures where complete conversion was not achieved in less than 1 h the total integrated area was obtained by heating the sample to approximately 50 "C above the reaction temperature and the additional FID response added to the area previously measured. The calibration factor, K,, was determined by using a high molecular weight paraffin wax, which in turn enabled absolute yields to be measured. Sample Preparation. The two samples of oil shale used in this work originated drom the Hartley Vale deposit of torbanite in New South Wales and the Rundle lamosite
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985 1119
Figure 2. Isothermal fractional conversion of kerogen as a function of heating time: (A)lamosite shale; (0) lamosite kerogen; ( 0 )torbanite shale.
the extracted oil shale was 16%, with an ash value of 7.5%. Although the interaction of HC1 and HF with the organic functional groups in kerogen may not always be negligible, Saxby (1976) reports that for "high r a n k kerogens the changes are not large. Elemental microanalysis of the samples yielded the following results on an ash free basis: lamosite 69.6% C, 8.6% H, 2.3% N, 5.0% S, and 14.5% 0 (by difference); torbanite 80.9% C, 9.3% H, 1.4% N, 2.2% S, and 6.2% 0 (by difference). Whole oils were obtained by modified Fischer assay at a heating rate of 5-8 "C min-I, with no sweep gas. Compound identification was accomplished either by GC/MS analysis of whole oils or by comparison with published passed retention parameters. In order to study functional group reactions during pyrolysis, samples of kerogen were sealed in 2.5 cm3 stainless steel pressure vessels and reacted at 425 "C for periods of up to 2 min in a fluidized sand bed and then rapidly quenched in ice. The remaining solid material was washed with chloroform and dried under vacuum, and infrared spectra were run in KBr disks at 1% concentration.
deposit in Queensland. The torbanite sample gave an ash value of 41% and an approximate Fischer assay of 200 L ton-l. Those values for the lamosite were 73% and 140 L ton-l, respectively. The soluble matter was removed by soxhlet extraction for 3 days with ethanol/toluene azeotrope. This accounted for 1.9% and 1.6% of the weight of the torbanite and lamosite, respectively. The kerogen fraction of the lamosite was isolated by ultrasonic HCl/HF digestion of the minerals using a process similar to that described by Sandel and Walcheski (1981). Remaining pyrite was partially removed by flotation on tetrabromoethane. The final yield of kerogen concentrate based on
Results and Discussion Isothermal Data. Shown in Figure 2 are representative plots of the isothermal fractional decomposition of the two shales and the mineral free lamosite kerogen as a function of retorting time. Clearly, in the temperature range 400-500 "C the rate of pyrolysis of the lamosite is greater than that of the torbanite, and the mineral-free material is faster than both. A more detailed analysis of the pyrolysis kinetics is presented in part 2 of this work (see Charlesworth, 1985). The variation in composition of the vapors at selected conversions for Rundle lamosite at 425 "C is shown in Figure 3. The chromatograms illustrate
1.0
0.8
0.6
a 0.4
02
0
2
4
6
8
10
t , min.
EXTRACTED RUNDLE SHALE
a. a=0.025
d
Figure 3. Representative chromatograms showing the conversion dependent variation in vapor composition during retorting of Rundle lamosite a t 425 "C. Peaks labeled 1 = 1-alkenes, 2 = n-alkanes, 3 = toluene, 4 = ethylbenzene, 5 = m- and p-xylene, 6 = o-xylene, 7 = 2,6,10-tri9 = prist-1-ene, 10 = prist-2-ene. methylundec-1-ene, 8 = 2,6,10-trimethyltridecane,
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
toluene/( C8+Cl6+CZ4)
20
~prist-t-ene/( 2 6
II
)
-04
lo-trlmethylundec-2-ene/( II
,I
tridecane/(
II ti
)
0 c; I C ,
-03
i 114Y
I -0.1
% CONVERSION (iooa)
Figure 4. Ratio of aromatic and isoprenoid species to the sum of C8, CI6, and CZ4n-alkanes plus 1-alkenes, and 1-alkene to n-alkane ratios, as a function of conversion a t 425 "C for Rundle lamosite.
the variation in products as the pyrolysis progresses. At 2.5% reaction the vapor is very rich in aromatic species, the most prominent being alkylbenzenes, and also isoprenoid alkanes and alkenes. Remarkably, there are only relatively small amounts of the n-alkanes and 1-alkenes that eventually constitute the bulk of the whole oil. After 10% conversion the vapor consists of pairs of n-alkanes and 1-alkenes, together with substantial amounts of aromatics and isoprenoids. The single most abundant compound in this latter class is prist-1-ene. On the basis of retention indices many of the remaining molecules appear
to be saturated cyclic or branched compounds, and work is in progress to positively identify these by GC/MS. As the conversion increases from 22 to 85%, the dominance by the 1-alkenes and n-alkanes is very pronounced, and the benefits of collecting separate fractions within this range appear to be slight. In Figure 4 the peak area ratios for several of the identified compounds are plotted as a function of conversion. These highlight the relatively rapid evolution of the aromatics and isoprenoids in the first few percent of reaction. The 1-alkene to n-alkane ratio varies in a manner dependent on both conversion and carbon number. In general the ratio appears to decline over the first 50% of reaction and then increases slightly toward the latter half of the process. Examples of the time dependency of the vapor composition obtained by pyrolysis of the Hartley Vale torbanite are shown in Figure 5. The most abundant single species after 30 s (5% conversion) is toluene; however, compared to the lamosite at a similar level of conversion there is a larger proportion of 1-alkenes and n-alkanes with the alkane predominating for all but the lowest carbon number (see Figure 6). After 3 min (25% conversion), the vapor consists essentially of only 1-alkeneln-alkane pairs in ratios always less than unity. No evidence for prist-1-ene is apparent at any stage, and the other isoprenoids also appear in much lower concentrations. Because of the relatively large proportion of oxygen in the lamosite (O/C = 0.16) compared to the torbanite ( O / C = 0.06), it is to be expected that the thermally induced destruction of functional groups should play a significant role in the overall degradation of the lamosite. Table I shows the major infrared absorption bands and their assignments for the two kerogens. The important functional groups are esters, amides, aldehydes, and carboxylic acids and their salts. Similar functional groups are known to be present in Green River shale, and Robinson (1969) and Burnham et al. (1982) have shown that during the pyrolysis of this shale the isoprenoid, saturated cyclic, and aromatic hydrocarbons are also evolved at low conversions. Consistent
EXTRACTED HARTLEY VALE SHALE TOLUENE
a. ~ o s E c . , ~ ~ o '
12
X4*X1
I xe+x
8
9
b. 3MIN.,450°
2
Figure 5. Representative chromatograms showing the variation in composition for the pyrolysis of torbanite a t 450 O C : (a) 5%; (b) 25% conversion. SCOT column (SE 30, 45 m X 0.5 mm, Ne, = 28,000) 50 ' C for 10 min, 4 O C min-' to 250 "C, N 2 flow rate 3 cm3 mi&.
1121
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985 O!
15c
,
W
Z
a
1
ll*
0
'
8
10
12
16
14
18
20
CARBON NUMBER
Figure 6. 1-Alkene to n-alkane ratios for torbanite pyrolysis: ( 0 ) 30 s, 450 "C; (A)3 min, 450 "C; (+) 9 min, 450 OC; (m) whole oil, 5-8 deg m i d . Table I. Infrared Absorbance Data for Kerogens intensity frequency, Hartley cm-' group Valeb Rundle W M 3400 NH, OH stretch S S 2930, 2850 CH2, stretch C=O, stretch (ester, amide, acid, M S 1700 aldehyde) 1620 C=O, stretch (carboxylate) NH, M S bend (amide) C-C stretch (aromatic) S M 1460 CH2, CH3, bend W W 1360 CH3, bend M M 710 CH2, rock W = weak, M = medium, S = strong. Measured from spectrum presented by Levy and Stuart (1983).
with this result is evidence that COzis evolved very early in the pyrolysis of Green River shale (Fester and Robinson, 1966). A similar phenomenon occurs during the heating of low rank coals, and functional group analysis has revealed that decarboxylation of acids is a major contributor (Murray and Evans, 1972). Approximately 11%of the product yield, based on the Fischer assay of this grade of Rundle shale, is COz (McFarlane et al., 1978). Furthermore, esters are also known to undergo a thermally induced &elimination reaction of the following type (Migrdichian, 1957). R'COOCH,CH,-R
-
+
R'-COOH
I R'-H
R-CHECH,
50
l-7
I
3000
This process appears particularly relevant since prist-1-ene is known to be formed during the pyrolysis of a phytanic ester (van de Meent et al., 1980b). Dehydration of amides by a similar process, to produce nitriles and alkenes, is an additional pathway that may be important. A homologous series of n-alkyl nitriles is present in the oil from Rundle shale (Regtop et al., 1982) but is absent in Hartley Vale oil (unpublished data). Figure 7 illustrates some examples of the infrared spectra of unheated lamosite kerogen concentrate and residual kerogen from samples that have been reacted to 10% and 22% conversion at 425 "C. The major changes in the spectra are the reduction in intensity of the bands at 1700 and 3400 cm-l. As these are characteristic of the major functional groups, it can be concluded that a large percentage of the oxygen- and nitrogen-containing groups are removed in the early part of the reaction.
800
1400
500
Figure 7. Infrared spectra of fresh lamosite kerogen and after 10% and 22% reaction at 425 "C. Table 11. 1-Alkene to n -Alkane Product Ratios for Vapor SamDles from Isothermal Oil Shale Pyrolyses Rundle Hartley Vale temp, "C % conversion
425 22
425 85
Molar Ratios 1.14 total, 1-alkene/n-alkane 1.02 (C,-Czs) C7H14/C7H16
C8H16/C8H18 C9H,s/C9Hzo ClOH20/C1OH22 CllHZZ/CllH24
C12H24/C12H26 C13H26/C13H28
1.14 1.24 1.04 1.20 1.03 1.06
1.22 1.31 1.13 1.08 1.12 1.19
1.03
1.01 0.88
C14H28/C14H30
0.73
C15H30/C15H32
1.10
C16H32/C16H39
1.04 1.01 0.97
C17H34/C17H36 C18H36/C18H38
1.00
1.30 1.18 1.15
1.13 1.23 1.02 1.05 1.02 1.06 1.09 1.05 1.22
C19H38/C19H40 C20H40/C20H42 C21H42/C21H44
0.93 0.82
C22H44/C22H46
0.88
C23H46/C23H48
0.85 0.96 0.86 0.83 0.90
0.91
1.04
0.85
C24H48/C24H50
C25H50/C25H52 C26H54
C27H54/C27H56
CO,
1700
WAVENUMBER ( C d )
C2eH52/
+
2000
C28H56/
C28H58
425 20
475 20
0.86
0.95
0.91 0.84 0.84 0.92 0.95 0.90 0.90
0.96 0.89 0.96 1.13 1.06 1.06 1.01 0.95
0.88 0.89 0.84 0.85 0.83
0.80 0.79
0.80 0.78 0.73 0.79 0.84
0.82 0.74 0.73
1.00 0.97 0.90 0.90 0.92 0.81 0.81 0.85 0.75 0.81
0.87 0.75 0.77
0.84
A comparison of the 1-alkene to n-alkane ratios in the vapors from the torbanite and lamosite at the same temperature and approximate conversion is presented in Table 11. The larger ratio in the lamosite vapors may be related to the 0-elimination reaction described above; however, if this is the case, then the alkyl group which also results must be liberated later in the process. This may explain the decline in the relative amount of 1-alkenein the middle stages of the reaction as shown in Figure 4. The rapid production of aromatic material is consistent with a recent study of the pyrolysis of model long-chain alkylbenzenes (Mushrush and Hazlett, 1982), which has shown that the rate of bond rupture is substantially greater than for the corresponding alkane and that scission occurs at a positon leading to toluene as the major product. Additional insight into the differences in behavior of the two types of kerogen can be gained by examining other
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
1122
I
I
I
-v-
5
1
-2 1 5
-, -
1
I
0
-
400
425
450
525
500
475
TEMPERATURE T ( ' C )
Figure 8. Plots of the normalized FID response and hydrocarbon yield as a function of temperature for lamosite kerogen (R,R') and at a heating rate of 10 deg mi&. torbanite (H,H')
data relating to the formation and structure of these materials. Microfossil remains show that the precursors of torbanite bear a strong resemblance to the freshwater and brackish-water colonial alga Botryococcus braunii, which is periodically abundant in the Coorong Peninsula of South Australia (Cane, 1967). These blooms contain substantial quantities of unsaturated linear and branched hydrocarbons that are capable of polymerization, by Diels-Alder condensation and through the allylic carbon atoms, to produce resinous deposits similar to natural rubber (Saxby, 1981). Green River shale has also been categorized as a lamosite, and chemical degradation studies have shown that carboxyl and ester groups represent 15% and 25% of the Green River kerogen oxygen, respectively, while a further 7% is accounted for as hydroxyl, carbonyl, and amide oxygen, with the remaining 54% probably occurring as ether bridges (Fester and Robinson, 1966). In the case of Rundle shale it appears that many of the carboxylic groups may be in their ionized form. Transmission elec-
tron microscopy (Glikson, 1983) has identified electron dense metal regions that are thought to be bound to humic acid components. Similarly, optical microscopy studies by the same worker show considerable surface interaction between the organic and inorganic components, with the mineral grains enveloped by the kerogen to form an interlocked composite. Model compound studies reveal that oxygen linkages are in general more thermally labile than polymethylene bridges (Cronauer et al., 19791, and thus the observed slower pyrolysis rate of the torbanite is to be expected. Nonisothermal Measurements. Figure 8 shows plots of the normalized FID response, and integral, for the lamosite kerogen and torbanite at a heating rate of 10 deg min-'. The differences observed in the isothermal experiments are reflected by the similar trends in these data. In particular the evolution of hydrocarbons from the torbanite takes place in the range 425-515 "C while the lamosite degrades in a range approximately 10-20 "C lower. Also shown in Figure 8 is the yield of the hydrocarbons, expressed as a percentage of the organic starting material. The method shows clearly that the yield is highest for the lamosite kerogen. The chromatograms illustrated in Figure 9 represent fractions of the vapors sampled at 425 and 465 "C during the pyrolysis of the lamoaite kerogen at 10 deg min-'. Qualitatively the trends observed in these data can be interpreted in a similar way to the results obtained by the isothermal approach. Specifically, the aromatic compounds are evolved in relative abundance at low temperatures together with prist-1-ene and other isoprenoid species. It should also be noted that unlike the undemineralized material, prist-2-ene is absent. At the upper temperature the vapors consist mostly of 1-alkenes and n-alkanes. The influence of temperature on the 1-alkene to n-alkane ratio can best be illustrated by the data in Table I1 for torbanite reacted to 20% conversion at 425 and 475 "C, respectively. In this system the production of alkenes and alkanes should not be complicated by contributions from functional group degradations, and the data clearly indicate that both the total and individual 1-alkene to n-alkane molar ratios increase as the temperature increases. This result can be compared with the data
RUNDLE KEROGEN PRIST-(&NE
425'
C,ALKY L BENZENES
6
r? TOLUENE
13 9
lo 11
X16 +X
16
2
Figure 9. Representative chromatograms showing the temperature dependent variation in vapor composition for pyrolysis of lamosite kerogen at 10 deg min-'. Analysis conditions as for Figure 5.
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
1123
Table 111. Frequency Factors and Activation Energies for Alkyl Radical Reactions Determined from Paraffin Pyrolysis Studies frequency factor, activation A , s-l or energy, E , cm3 mol-' s-l kJ mol-' reference reaction
transfer CH3. C4Hlo -+ C2HE. + C4Hio C3H7. + C3H8 -+ CkHg. + C2H6
+
CH4 + C4H9. C& C4Hg' C3H8 + C3Hy C4H1o + CZHS.
+
4
+
3.5 x 1013 1.6 X 1O'O 6.5 X 10" 3.2 x 1013
131 92 100 109
Lin and Laidler (1966) Kerr and Trotman-Dickenson (1960) Gruver and Calvert (1956) Lin and Back (1966)
3 x 1014 1.3 x 1014 1 x 1014
165 155 167
Laidler and Wojciechowski (1961) Laidler et al. (1962) Lin and Back (1966)
2.7 X 10" 7.7 x 10" 1.3 X 10l2 5 x 10"
38 44 71 63
presented by Coburn et al. (1978), Burnham et al. (19821, and Saxby and Riley (1984),which show that the 1-alkene to n-alkane ratio depends strongly on the rate of heating. As the rate increases, a greater proportion of the degradation occurs at the higher temperatures, and this is the factor which influences the distribution. The reactions leading to alkane and alkene production can be summarized as follows: kerogen R1-(radical formation)
--
R1. R2. (isomerization)
R2.
kdor
kd
alkene
Jones and Steacie (1953) Boddy and Steacie (1960) Leathard and Purnell (1968) Lin and Back (1966)
Table IV. 1-Alkene to n -Alkane Product Ratios for the Cracking of n -Hexadecane at 68 atmD temp (wall), OC 610 704 temp (effective), OC 570b 610 % conversion 68.8 64.8 % unidentified products 20.4 13.6 Molar Ratios total, 1-alkene/n-alkane (Cz-Ci3)
C12H24/C12H26
1.05 0.73 1.27 1.50 1.3 1.5 1.5 1.5 1.3 1.5
C13H26/C13H28
1.4
C2H4/C2H6 C3H6/C3H8 C6H12/C6H14 C7H14/C7H16
+ R3- (or He)(fission or H-loss)
+
R2-(or R1.,R3-,etc.) + H-donor alkane
kT
CllH22/CllH24
H-donor -H. (transfer)
In this process the decomposing kerogen first produces a large primary generation free radical that may intramolecularly rearrange to a thermodynamically more stable form. Fission or H-loss occurs, and an alkene and a smaller second generation free radical or H atom are formed. The isomerization and fission steps may occur several times before the remaining pathway, involving hydrogen abstraction, converts the radical into a saturated species. The material that has donated the hydrogen may continue the chain process or terminate by recombination with other radicals in the system. It is difficult to predict the product distribution, even for a simple case involving a single saturated hydrocarbon, because of the potential for more than one isomerization and fission step (Fabuss et al., 1964). Fortunately the effect of temperature, as pointed out by Raley (19801, can be estimated by considering the probable variation in the rate constants for these reactions. It follows from the above scheme that the rates of production of alkene and alkane are given by the simplified expressions
d[alkane] = k~[alkyl*] [H-donor] dt Therefore the relative rate of production is d[alkene] --kF + k H 1 d [alkane] kT [H-donor]
C8H16/ C8H1.9 C9H18/C9H20 ClOH20/ClOH22
(5)
(6)
Table I11 lists a selection of frequency factors ( A )and activation energies ( E )determined for specific radical re-
1.51 1.21 1.97 1.71 1.4
1.8 1.7 1.9 1.6 1.6 1.5
C4 not reported, C5 inaccurate nCalculated from the data presented by Fabuss et al. (1962). Interpolated value.
actions encountered in paraffin pyrolysis studies previously published in the literature. Using average values for these quantities enables kF, kH,and k T to be calculated at any temperature by application of the Arrhenius expression
k = A exp(-E/RT)
(7)
where R is the gas constant. Hence the relative variation of the 1-alkenes and n-alkanes with temperature, at any conversion, is related to the temperature dependence of the quantity (kF kH)/kT. An empirical test of the validity of this approach, as it applies to paraffin pyrolysis, can be made by reexamining the data presented by Fabuss et al. (1962) for the cracking of n-hexadecane at 570 and 610 "C under a pressure of 68 atm. Table IV lists the main product ratios calculated from the results presented by these workers. Note that both the total and the individual 1-alkene to n-alkane ratios increase with temperature in the expected direction by factors ranging from 1.1to 1.66, with the total ratio increasing by 1.44. The rate constant ratio, calculated from the data in Table I11 and eq 7 , increases by a factor of 1.42 as the temperature changes from 570 to 610 "C. The magnitude of this change is consistent with the compositional variation. The effect of heating rate on the present oil shales is best illustrated by comparing the whole oils produced by Fischer assay retorting (5-8 deg min-l) with the vapors from the 450 "C isothermal pyrolysis of Hartley Vale torbanite. The 1-alkene to n-alkane peak height ratios for
+
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
the samples are plotted in Figure 7. It is apparent from these data that the Fischer assay sample contains a much smaller proportion of 1-alkenes and that there is not a large change in the ratio with conversion for any particular carbon number. A more obvious demonstration of the effect of temperature is highlighted by the variation in composition of the 13 fractions collected during the slow heating (0.03 deg min-I) of an 18-cm block of Green River shale (Coburn and Campbell, 1977). These data have been recently reanalyzed by Burnham et al. (1982) and show a steady increase in the 1-alkene to n-alkane ratio progressing from the low- to high-temperature fractions. Temperature may not be the only parameter influencing oil composition, as the presence of an inert diluent in the form of the sweep gas has also been reported to change the 1-alkene to n-alkane ratio under certain retorting conditions (Bumham and Ward, 1981). In terms of the previous reaction scheme, the effect arises because the relative reaction rate is inversely proportional to the concentration of hydrogen donor species, which are in theory diluted by the sweep gas. Oils prepared by Fischer assay retorting are normally not subjected to sweep gas, and it is suggested that this is a reason for the low ratios. This argument assumes that the reaction is essentially homogeneous and that the hydrogen donor molecules are also in the gaseous phase. Undoubtedly some of the reactions leading to the final products do occur as a result of this type of process. Supporting evidence for this comes from the distribution of aromatic compounds, which is significantly different in the Fischer assay oils compared to the rapidly pyrolysed samples (Burnham and Ward, 1981). Gas-phase condensation reactions involving mono- and dialkenes have been shown to lead to these materials (Fabuss et al., 1962;Sakai et al., 1970). However, it seems probable that the kerogen itself, which is present as a condensed phase and is not removed from the site of production of the radicals, should act as a major hydrogen donor. Results presented by Voge and Good (1949) for the gas-phase cracking of n-hexadecane at 500 OC show that a pressure of only 1 atm the 1-alkene to n-alkane ratios are very high, ranging from 1.5 for C2 to 15 for C5at 42% conversion. Thus the secondorder reaction is suppressed because the concentration of hydrogen donors at these pressures is greatly reduced. Provided this hypothesis is correct, a major effect of the sweep gas is to remove the products from the hot zone and reduce the opportunity for secondary high-temperature reactions. The most significant point that arises when considering the role of the unreacted kerogen is that during isothermal pyrolysis the relative rate of production of alkenes and alkanes as defined by eq 6 should increase in proportion to the increase in conversion because the concentration of hydrogen donors is steadily diminishing. The ratio results presented in Table I1 and Figures 4 and 6 do not support this argument and in fact show that after the initially high levels of 1-alkene, possibly arising from the functional group degradations, the ratio remains fairly constant. This is indicative of a heterogeneous reaction of the type known to be responsible for the thermal decomposition of many crystalline organic solids (Young, 1966). In these materials the reactants are essentially immobilized, causing the reaction to occur at definable points and then to proceed by growth into the surrounding matrix of undegraded material. Hence the reaction can be described in terms of the laws governing the growth of nuclei, with the reaction occurring preferentially at a phase boundary as a consequence of the lower coordination energy at this location. In the case of kerogen decomposition at the interface be-
tween the two regions, the concentration of hydrogen donors is expected to be approximately constant, and therefore the alkene and alkane production rates should not vary greatly as the nuclei consume the kerogen. The overall kinetics of the pyrolysis process described in part 2 of this work show that a rate law consistent with a heterogeneous mechanism fits the experimental data. Summary and Conclusions The time- and temperature-dependent variations in the composition of the oil produced during the laboratory retorting of oil shale can be simply monitored by on-line sampling of the retort vapors followed by direct analysis by capillary gas chromatography. In agreement with results obtained by other workers, using nonisothermal methods, the production of aromatics, isoprenoids, and saturated cyclic and branched material occurs early in the reaction. The vapor is relatively uniform in composition over the remainder of the reaction. The inidividual 1alkene to n-alkane ratios are high at short times and low conversions,and this is interpreted in terms of the thermal destruction of specific functional groups, particularly esters and amides. The magnitude of the change in the vapor composition is dependent on the type of kerogen undergoing pyrolysis and is high for kerogen containing large amounts of 0, N, and S. A reexamination of the temperature dependency of the 1-alkene to n-alkane ratios obtained previously for pyrolysis of n-hexadecane at high pressures shows that the predicted relative reaction rates of alkyl radicals is observed. The temperature or heating rate effects on the 1-alkeneto n-alkane ratios in the present work agree qualitatively with this behavior and support conclusions reached by other workers. However, the evidence also suggests that the abstraction reaction leading to the alkanes occurs mostly within the kerogen rather than in the gaseous phase. Furthermore, the independence of the composition with time in .the middle and later stages of the conversion implies that the pyrolysis proceeds by a heterogeneous mechanism involving an interface between reacting and unreacted regions. Acknowledgment
I would like to thank G. Kelso and L. Beranek for providing the Fischer assay analyses. Literature Cited Boddy, P. J.; Steacle, E. W. R. Can. J. Chem. 1960, 38, 1576. Burnham, A. K.; Ward, R. L. In "Oil Shale, Tar Sands and Rehted Materials" Stauffer, H. G. Ed., ACS Symposium Series No. 163: American Chemical Society: Washington, DC 1981. Burnham. A. K.; Clarkson, J. E.: Singleton, M. F.; Wong, C. M.; Crawford, R. W. Geochlm. Cosmochlm. Acta 1982, 46, 1243. Cane, R. F. Proc. Seventh World Petroleum Congr. 1967, 3, 681. Charlesworth, J. M. Ind. Eng Chem. Process Des. Dev. 1985, following paper In this Issue. Coburn, T. T.; Campbell, J. H. Lawrence Livermore Natl. Lab. Report., UCRL52256 Part 2, Livermore, CA. Sept 8, 1977. Coburn, T. T.: Bozak, R. E.; Clarkson, J. E.; Campbell, J. H. Anal. Chem. 1978, 50,958. Cronauer, D. C.: Jewell, D. M.: Shah, Y. T.: Modi, R. J. Ind. Eng. Chem. Fundam. 1979, IS, 153. Dletz, W. A. J. Gas Chromatogr. 1967, 5. 68. Earnest, C. M. Thermochim. Acta 1962, 58, 271. bit,R. I.; Borsanyl, A. S.;Satterfleld, C. N. Ind. Fabuss, B. M.; Smith, J. 0.; Eng. Chem. Process Des. Dev. 1962, 4, 293. Fabuss, 8.M.: Smith, J. 0.; Salterfield, C. N. In "Advances in Petroleum Chemistry and Refining"; McKetta, J. J., Ed.: Interscience: New York, 1964. Faulkner, 8. P.; Weinecke', M. H. U.S. Patent 4405438, 1983. Fester, J. I.; Robinson, W. E. In "Coal Science"; Gould, R. F.. Ed.; American Chemical Society: Washington, DC, 1966. Glikson, M. "Proceedings, First Australian Workshop on Oil Shale"; Lucas Heights, May 1983, p 39. Gruver, J. T.; Calvert, J. G. J. Am. Chem. SOC.1956, 78,5208. Jones, M. H.; Steacie, E. W. R . Can. J. Chem. 1953. 31, 505. Kerr, J. A.; Trotman-Dlckenson, A. F. J . Chem. SOC.1960, 1602. Laidler, K. J.; Wojciechowski, B. W. Proc. R . SOC.London A 1961, 260, 91. Laidler, K. J.; Sagert, N. H.; Wojciechowski, B. W. Proc. R . SOC.London, A 1962, 270, 242.
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Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 1125-1132 Leathard, D. A.; Purneil, J. H. R o c . R . SOC. London A 1968, 306, 553. Levy, J. H.; Stuart, W. I.Proceedings First Australian Workshop on Oil Shale, Lucas Heights, May 1983, p 135. Lin, M. C.; Back, M. H. Can. J . Chem. 1966, 4 4 , 2389. Lin, M. C.; LaMler, K. J. Can. J . Chem. 1966, 4 4 , 2927. McFarlane, I.; Rankine, J.; Walker, D. R. Southern Pacific Petroleum Report to Shareholders, Sydney, NSW, April 27, 1979. Miirdichian, V. “Organic Synthesis-Volume 2”; Reinhoid: New York, 1957. Murray, J. B.; Evans, D. G. Fuel 1972, 57, 290. Mushrush, G. W.; Haziett, R. N. Naval Research Lab. Report No. 8630 Washington, DC, Sept 21, 1982. Raley, J. H. Fuel 1980, 59, 419. Regtop, R. A.; Crisp, P. T.; Ellis, J. Fuel 1982. 67, 185. Robinson, W. E. I n “Organic Geochemistry”; Eglinton, G.; Murphy, M. T. J., Ed.; Springer-Verlag: Berlin, 1969. Robinson, W. E. I n “Oil Shale”; Yen, T. F.; Chiiingarian, G. V. Ed.; Elsevier: Amsterdam, 1976. Sakai, T.; Soma, K.; Sasaki, Y.; Tominga, H.; Kunugi, T. I n “Advances in Chemistry No. 97, Refining Petroleum for Chemicals”; American Chemical Society: Washington, DC, 1970.
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Sandei, R. S.; Waicheski, P. J. Fuel 1981, 6 0 , 644. Saxby, J. D. I n “Oil Shale”; Yen, T. F.; Chiiingarian, G. V., Ed.; Elsevier: Amsterdam, 1976. Saxby, J. D. fuel 1981, 60, 994. Saxby, J. D.; Riley, K. W. Nature (London) 1984, 308, 177. van de Meent, D.;Brown, S. C.; F’hiip, R. P.; Simoneit, B. R. T. Geochim. Cosmochim Acta 1980, 44, 999. van de Meent, D.; de Leeuw, J. W.; Schenck, P. A. I n “Advances in Organic GeOChemlStry 1979”; Douglas, A. G.; Maxwell, J. R.; Ed.; Pergamon: London, 1980. Voge, H. H.; Good,G. M. J . Am. Chem. SOC. 1949, 7 1 , 593. Wallman, P. H.; Tamm, P. W.; Spars, B. G. I n “Oil Shale, Tar Sands and Related Materials”; Stauffer, H. C., Ed.; Symposium No. 163; American Chemical Society: Washington, DC, 1981. Weichman, B. E.; Knight, J. H. US. Patent 4 133 741, 1979. Young, D. A. “Decomposition of Soilds”; Pergamon: London, 1966.
Received for review March 12, Revised manuscript received October 23,
1984 1984
Oil Shale Pyrolysis. 2. Kinetics and Mechanism of Hydrocarbon Evolution John M. Charlesworth Materials Research Laboratories, P.0. Box 50, Ascot Vale, Victorla, 3032 Australia
The evolution of hydrocarbons from a variety of oil shales and kerogen concentrate has been measured by using flame ionization detection, with the aim of monitoring the kinetics under both isothermal and nonisothermal cond%ions. The isothermally measured behavior shows that a single rate law does not operate at temperatures below 500 O C . Above 500 OC the interpretation of data is complicated by the delay in achieving thermal equilibrium before appreciable reaction occurs. The application of some of the more commonly used rate expressions for soli-phase decompositions suggests that the mechanism progresses from a diffusion-controlled reaction, through a phaseboundary process, to a reaction governed by nucleation and growth. Removal of the minerals does not appear to change the mechanism, but the rate of reaction is increased. I n the case of nonisothermal measurements at heating rates from 1 to 40 deg min-’, the kinetics can be adequately described by a simple first-order rate expression, together with the appropriate time-temperature transformation. The discrepancy between the two techniques is discussed in terms of the approximations and inaccuracies involved in measuring and processing the results.
Introduction The rate of thermal decomposition of oil shale has been studied extensively in recent times, and a review of the topic, covering work prior to 1978, has been presented by Rajeshwar et al. (1979). Since then a number of other significant contributions to the area have been published, including investigations by Campbell et al. (1978, 1980), Shih and Sohn (1980), Noble et al. (1981), Wallman et al. (1981), and Rajeshwar and Dubow (1982). Most recently a comparison of the pyrolysis behavior of key oil shales from eight countries has been presented by Nuttall et al. (1983). Due to the complexity of the chemical structure of kerogen, a mechanistic theory capable of describing the fundamental bond rupture and propagation steps at elevated temperatures is still not available. Because of this, most of the relationships that have been developed to describe the kinetics have essentially phenomenological foundations. Furthermore, there appears to be substantial disagreement among many of the proposed models, particularly in terms of those parameters that are normally considered to characterize the reaction. For example, measurements of activation energy for the decomposition range from approximately 20 to 240 kJ mol-’ in the 15 examples cited by Rajeshwar et al. (1979). Nuttall et al. 0196-4305/85/1124-1125$01.50/0
(1983)report that for Rundle oil shale the activation energy varies from approximately 30 to 160 kJ mol-l, depending on the method of analysis of the experimental data. In general, expressions involving first-order kinetics are the most popularly applied rate laws; however, Rajeshwar and Dubow (1982) have recently fitted a variety of more complex equations to the pioneering kinetic data produced by Hubbard and Robinson (1950). Variations such as these are partially attributable to the different reaction schemes, which may include up to 10 separate steps (Shnackenberg and Prien, 1953). Many of the intermediate species and products, such as bitumen, oil, and gas, are poorly defined in terms of their chemical identity, and this almost certainly leads to confusion when interpreting results. Other problems stem from the lack of uniformity in the experimental procedures adopted by various workers. In particular, particle- and sample-size variations can lead to differences in the rate at which the shale achieves thermal equilibrium with the surroundings. This may become critical at high temperatures where the rate of reaction can be faster than the rate of heating. Recently Rajeshwar and Dubow (1982) criticized the approaches that had been adopted in previous studies on the fundamental theoretical grounds that the pyrolysis of oil shale kerogen involves the 0 1985 American Chemical Society
