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A Pyrolysis-Gas Chromatographic Analysis of Athabasca Bitumen R. George S. Ritchle, Rodney S. Roche,' and William Steedman' Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4
A study of the pyrolysis of Athabasca bitumen has been made using the pyrolysis-gas chromatographic technique. At temperatures below 500 O C the evolution of volatile products from bitumen is dominated by a distillation process. Pyrolysis becomes more important in the higher temperature regions; by 950 OC deep-seated degradation to gases and light ends is dominant. The apparent activation energy of the low-temperature process is reported.
Introduction As part of a program concerned with the in situ recovery of the Athabasca tar sands we are engaged in a study of the pyrolytic behavior of bitumen extracts. We are prompted to report some significant early results by a recent paper concerned with the thermal degradation of an Orinoco bitumen (Fernandez Lozano et al., 1978). The pyrolysis of tar sand bitumen is regarded as a key step in the in situ wet or dry combustion recovery process, models for which have been described by a number of authors (Tadema, 1959; Flock and Dranchuk, 1971; Redford, 1976). The implied importance of bitumen pyrolysis has resulted in a limited number of useful studies of the pyrolysis of Athabasca tar sands (Speight, 1970; Bennion et al., 1977). There is, however, a severe lack of fundamental data relating to the mechanism of tar sand pyrolysis and the products obtained therefrom and to the kinetic and thermodynamic parameters associated with the hightemperature reactions. This communication reports the results of a pyrolysis-gas chromatographic study of bitumen pyrolysis which has been carried out prior to a kinetic analysis of the reaction. The pyrolysis-gas chromatographic technique has been the subject of a good review (Perry, 1968). It is most commonly applied to studying the pyrolysis of synthetic polymers but has also been used to examine pyrolytic reactions in fossil fuels including bitumens (Ariet and Schweyer, 1965; Marschner et al., 1973). Experimental Section Material. The Athabasca bitumen used in this work was supplied by Professor D. W. Bennion (University of Calgary). It had been extracted from whole tar sand, using toluene as solvent. The toluene content of the extract was estimated by infrared analysis to be less than 0.5% M. Prior to extraction the tar sand was stored at -10 "C. The extract was also stored at -10 "C, but it was warmed to room temperature before being applied to the pyrolysis probe. Separate maltene and asphaltene fractions were supplied by Dr. 0. Houwen (University of Calgary). They had been prepared by the precipitation of the asphaltene fraction from a benzene solution of Athabasca bitumen, using pentane as precipitant. Pyrolysis-Gas Chromatography. The pyrolysis unit used was a Pyroprobe 120 (Chemical Data Systems, Ltd.) interfaced with a Hewlett-Packard 5840A gas chromatograph. The pyrolysis probe consisted of a platinum coil into which was inserted a small open quartz boat conDepartment of Chemistry, Heriot-Watt University, Edinburgh
EH 14 4AS, United Kingdom. 0019-7890/78/1217-0370$01.00/0
taining the sample. The sample size was substantially below the milligram range and was thinly spread on the bottom of the boat. Maximum ramp times, giving heating rates of nominally 20000 OC/s were used, and the maximum set temperature was held for 20 s. Under these conditions isothermal flash pyrolysis conditions were assumed. The high degree of reproducibility of the pyrograms obtained indicated that diffusion of products from the hot zone was rapid and that secondary reactions, therefore, were minimized. Chromatographic analyses were carried out using a 6 f t X '/8 in. stainless steel column packed with 3% Dexsil400 GC on 80-100 mesh Chromosorb W-AW-DMCS. The column was programmed between 60 and 300 "C, rising at 10 OC/min after an initial 2-min period. The carrier gas was nitrogen flowing a t 30 mL/min, and effluent was monitored using a flame ionization detector. The outstanding stability and lack of column bleed exhibited by the Dexsil column permitted satisfactory analysis on single column mode. Thermal Volatilization Analysis (TVA). Activation energies for the pyrolysis of bitumen and of asphaltene in vacuo were measured by TVA. This technique has been described in detail elsewhere (McNeil, 1967). Essentially it consists of heating a sample to elevated temperature at a carefully controlled heating rate while pumping on the sample continuously. Volatiles evolved are passed to a cold trap while the pressure changes prior to the trap are monitored by a Pirani gauge. The plot of gauge response against sample temperature during volatile evolution effectively measures the rate of volatilization, which in turn is related to the rate of the degradation reaction. By carrying out the experiment at a series of different heating rates, kinetic data can be extracted from the TVA plots (Roche, 1974). Results and Discussion A series of bitumen pyrograms for temperatures ranging from 200 to 950 OC are shown in Figure 1. There are clearly three regions in each pyrogram which figure prominently or otherwise, depending on the temperature of pyrolysis. These may be described as follows. (i) The gas region is the family of peaks observed up to about 2.5 min. On the basis of comparison with the retention time of standard hydrocarbons these peaks correspond to alkanes and alkenes up to about c8, along with light boiling aromatics and, presumably, the noncondensable gases (H2,CO, COz, COS, H2S) which have been reported by other workers (Bennion et al., 1977). A more detailed analysis of the early gases will be carried out in due course, but some runs using a 5A molecular sieve loop attached to the main analytical column have indicated that methane is the major hydrocarbon component in the gas fraction under these conditions of pyrolysis. It will be
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 4, 1978
Table I. Variation o f "Gas" Peak (as % Volatiles) with Temperature 400 500 600 pyrolysis temperature, C 13.42 gas peak (% volatiles) 0.0 3.27
700 37.74
O
Table 11. Variation of Envelope Region (as % Volatiles) with Temperature 600 pyrolysis temperature, C 400 500 66.7 envelope region (% volatiles) 97.24 87.38
seen that a substantial gas peak is not observed until a pyrolysis temperature of 500 "C is used. Thereafter the gas peak steadily increases both in size and in complexity with increasing temperature, as indicated in Table I. (ii) The fingerprint region is the region between 2.5 min and about 14 min. Like the gas region, the fingerprint region is dormant until a pyrolysis temperature of 500 "C is reached. In the higher temperature pyrograms a highly complex pattern characteristic of the pyrolysis temperature, and quite reproducible under carefully controlled conditions, is obtained. A t intermediate temperatures (600 "C) the region has a certain regularity which extends beyond the 14 min retention time and which, we believe on the basis of retention time data of standards, represents the n-alkane homologous series. Not unexpectedly this series largely disappears at higher temperatures, where further cracking and isomerization reactions will occur. (iii) The envelope region is the region between about 14 min and 30 min. This region has an inverse thermal history to that of the other two regions, being dominant at 200 "C,then decreasing with increasing temperature as the material it represents is pyrolyzed to smaller molecules. At a pyrolysis temperature of 950 "C the envelope region has virtually disappeared. Table I1 quantifies the variation of the envelope with temperature. The most significant conclusion to be drawn from these observations is that gross pyrolysis does not occur until a temperature of about 500 "C is reached. (It must be remembered that the temperature referred to is that of the pyrolysis probe. It is not possible to measure directly the temperature of the minute sample at the time at which loss of volatiles occurs. Under the conditions used the sample will receive heat from the coil mainly by radiation; nevertheless a temperature lag must occur, and the "pyrolysis temperature" quoted is likely to be higher than the true sample temperature in the initial stages of devolatilization.) The evidence for this is that a negligible gas zone and no fingerprint region is observed below 500 "C. The envelope we attribute not to a pyrolysis phenomenon but to a distillation phenomenon. This thesis was tested by "pyrolyzing" a maltene saturate fraction, of average carbon number ca. Cm, at 500 "C and chromatographing the same fraction by direct injection. The resulting chromatograms were qualitatively very similar and consisted only of envelope regions corresponding to those shown in Figure 1. It appears that pyrolysis at 500 "C did not degrade the maltene but simply distilled the material into the analytical column. In situ combustion recovery processes are operated at temperature maxima in the 400-500 "C range. It appears, therefore, that distillation may play an even more important role, and pyrolysis a much less important role, in the in situ process than has been previously supposed. To test the effectiveness of the distillation process in removing volatiles from the pyrolysis zone, a bitumen sample was successively pyrolyzed at 500,700, and 950 "C without the sample and probe being removed between runs. The pyrograms recorded are shown in Figure 2. Evidently very little volatilizable material is left after the initial heating at 500 " C .
950 61.59
800 41.90
800 45.08
700 51.37
O
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950 10.52
PYROLYSIS TEMP 400°C
I
PYROLYSIS TEMP
500°C
PYROLYSIS TEMP 600°C
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W l z n
0 a lW n LL 0 k W 0
c 0 W
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II
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I 10
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1
I
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RETENTION TIME, min
Figure 1. Variation of bitumen pyrograms with pyrolysis temperature (GC column temperature profile indicated).
Separate pyrolysis at 500 "C of asphaltene and maltene fractions have also been recorded. The interesting results are shown in Figure 3, which indicates (a) that the distillation process is largely attributable to the maltene fraction (which in any case represents about 85% of the total bitumen) and (b) that the pyrolysis of asphaltene at 500 "C produces at least one (and perhaps more) homologous series of compounds through the fingerprint region. The pyrogram at 600 "C, therefore (Figure l),may represent a large distillation envelope derived from the maltene fraction, preceding and superimposed on which
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PYROLYSIS T E M P 5 0 0 ' C
I
2
J
PYROLYSIS TEMP 7 0 0 ' C
I
PYROLYSIS T E M P 950.C
II
I
5
1
I
I
1
25
30
I
?O 15 20 RETENTION TIME, min.
Figure 2. Pyrograms for successive bitumen pyrolyses a t 500,700, a n d 950
OC.
precision. Rather it appears that there is a continuous change of emphasis from distillation to pyrolysis as the sample temperature increases. In the real situation the residence time of organic material in a hot zone will play an important part in determining whether distillation, isomerization, or cracking predominates. Activation energies for the pyrolysis of whole bitumen and of asphaltene were measured using the TVA technique. Kinetic data obtained from thermoanalytical techniques applied to complex reaction systems must be notional but may be very useful in modelling such systems. Over the temperature range ambient to 440 "C the apparent activation energies obtained were for bitumen, 45.98 kJ/mol and for asphaltene, 188.10 kJ/mol. The figure obtained for asphaltene is of the same order as that reported for, e.g., coal (Chermin and van Krevelen, 1957) and for shale oil (Braun and Rothman, 1975) and is typical of a condensed involatile hydrocarbon phase undergoing thermal degradation by a homolytic, free-radical process. The low activation energy obtained from bitumen, however, could not be ascribed to a homolytic degradation reaction of hydrocarbon material, and in fact is essentially a reflection of the latent heat of volatilization of a sample undergoing distillation rather than pyrolysis. It is of interest to note that the Orinoco study reported without comment similar large variatiods in activation energy. Conclusions Athabasca bitumen undergoes distillation, but not pyrolysis, when flash heated at atmospheric pressure at temperatures up to about 500 "C. Thereafter, as the temperature is raised distillation competes with pyrolysis until, at temperatures of the order of 950 "C, the latter process dominates. The distillation process is dominated by the maltene fraction, whereas the asphaltene fraction does not distill but begins to exhibit degradation patterns a t temperatures upward of 500 "C. A more detailed analysis of the products of the distillation/pyrolysis model is being actively pursued and will be reported in due course along with further relevant kinetic data.
Acknowledgment The authors wish to express their thanks to AOSTRA (Alberta Oil Sands Technology and Research Authority) for supporting this work. Literature Cited I 5
IC
I5 20 RETENTION TIME, min
25
30
Figure 3. Pyrograms for the pyrolyses a t 500 " C of (a) asphaltene and (b) maltene.
is a series of volatile pyrolysis products, including a homologous series, derived largely from the pyrolysis of asphaltene. The proposed distillation/pyrolysis model is broadly in agreement with that recently outlined for Orinoco bitumen (Fernandez Lozano et al., 1978). These latter workers suggested a change of mechanism from predominantly distillation to predominantly cracking "at about 324 "C". In practice, it is not possible to define a mechanistic change from a physical to a chemical process with any sort of
A h t , M., Schweyer, H. E., I&. Eng. Chem. Rod. Res. Dev., 4, 215-220 (1965). Braun, R. L., Rothman, A. J., Fuel, 54, 129-131 (1975). Cherrnin, H. A. G., van Krevelen, D. W., Fuel, 38, 85-104 (1957). Fernandez Lozano, J. A,. Gonzalez, A. R., Fernandez, C. M., Ind. Eng. Chem. Prod. Res. DW., 17, 71-79 (1978). Flock, D. L., Dranchuk, P. M., J. Can. Pet. Techno/., 33-39 (1971). Hayashitani. M., Bennion. D. W., Donelly, J. K., Moore, R. G., "Thermal Cracking of Athabasca Bitumen", Canada-Venezuela Oil Sands Symposium, Edmonton, 1477
Marychnk, R. F., Dum, L. J., Winters, J. C., Am. Chem. Soc. Div. Pet. Chem. P r e p . , 18 (3), 572-583 (1973). Perrv. S.G.. Adv. Chromatour.. 7. 221-241 119681. Redford, D.'A., Chem. Can.,-20-23 (Sept 1976). ' Roche, R . S., J . Appl. Polym. Sci., 18, 3555-3569 (1974). Speight, J. G., Fuel, 49, 134-145 (1970). Tadema, H. J., Proceedings of the 5th World Petroleum Congress Sectlon 11, Paper 22, 1959.
Received for review July 5 , 1978 Accepted August 21, 1978
