Energy & Fuds 1987,l , 501-506
carbon black and, by doing so, reduces oil yield. Techniques that shift the product distribution from gas toward liquid may raise the molecular weight of heavy liquid components and result in nonvolatile products (coke). Burnt shale promotes coking.18 Flash pyrolysis and a rapid gas sweep (fluidized-bed conditions) keep coke formation to a minimum and so provide the opportunity to reduce gas production, increase oil yield, and still avoid (18) Levy, J.
H.;Mallon, R. G.; Wall, G. C . Fuel
501
excessive losses due to coking. The task remains to define the limiting conditions. Acknowledgment. Gas chromatography was performed by J. E. Clarkson and J. Cupps. This contribution and help from members of the LLNL oil shale program, including A. K. Burnham, J. G. Reynolds, R. W. Taylor, K. G. Foster, and G. J. Koskinas, are gratefully acknowledged. Work was performed by the Lawrence Livermore National Laboratory for the U.S.Department of Energy under Contract No. W-7405-ENG-48.
1987, 66, 358-64.
ESR Studies of Kerogen Conversion in Shale Pyrolysis B. G. Silbernagel,* L. A. Gebhard, M. Siskin, and G. Brons Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received June 22, 1987. Revised Manuscript Received August 28, 1987
ESR studies have been performed on a series of Green River and Rundle oil shale samples that have been pyrolyzed at 350 and 375 OC for time scales on the order of several hours. The carbon radical density in the residual shale increases with increasing thermal treatment, growing by as much as a factor of 5 for the Rundle shales. This radical increase is accompanied by a fall in the g value and a decrease in the width of the carbon radical ESR absorption. The former is attributed to the generation of aromatic radical anions, for which g 2.0028. This tends to counter the effect of the starting radicals, which, due to the presence of heteroatoms, may have higher g values. The latter is believed to be associated with the depletion of hydrogen-rich alkyl side chains during the pyrolysis process. Since the carbon radical line width arises from hyperfine interactions of the radical's unpaired electrons with adjacent protons, the reduction in proton density resulting from the dealkylation could account for this decrease in density.
-
Introduction The present paper describes the use of electron spin resonance (ESR) to trace the changes that occur in shale kerogen during the course of pyrolysis a t relatively low temperatures (-350-375 OC). Since carbon bond breaking is a key feature of such a pyrolysis process, we would anticipate that a remnant of this chemistry would be preserved in the carbon radicals, molecules bearing unpaired electrons, which remain behind in the residual organic component after the completion of the thermal treatment. The character of these radicals can be established by examining the ESR properties of the radicals: their g value, the line width of the ESR absorption, the number of radicals that remain, and their response to microwave fields of varying intensity (microwave saturation), with particular emphasis on changes in the character of these radicals with increasing extent of treatment. Marchand and co-workers' were the first to study kerogens with ESR. The paramagnetic susceptibility (i.e. the number of radicals) and the line width were studied as functions of hydrocarbon evolution due to burial and of the temperature of pyrolysis (300-600 "C). They investigated four different types of kerogens and the chloroform-extracted kerogen concentrates. Good agreement between results on the natural and pyrolyzed shales were D
(1) Marchand, A.; Libert, P.; Combaz, A. C.R. Seances Acad. Sci., Ser. 1968,266, 2316-19; Rev. Znst. Fr. Pet. 1969, 24, 3-20.
obtained. They found that the number of radicals generally increases as a function of evolution and is a function of the organic matter type from which it is derived. Fluctuations of the line width were too great to permit correlation with evolution or for differentiating kerogen types. Contrary to the conclusions of Pusey2 they cond u d e d that the measurement of radical density cannot determine a catagenic evolution stage by itself. However, they concluded that, combined with vitrinite reflectance, kerogens can be classified according to both type and degree of evolution. On the basis of the previous results, Hwang and Pusey2 proposed the use of g values, line width, and spin density data on kerogen concentrate samples only, for evaluating the thermal history of a formation and, from this, the level of maturity of petroleum-forming sediments. Kaplan et aL3 separated the kerogen from a young marine sediment (Tanner Basin, offshore California) and heated it in sealed glass tubes at a single temperature between 150 and 410 "C for a specific time in the range of 5-120 h. With increasing degree of thermal alteration of the kerogen (decreasing H/C), spin density increases up to a value of 0.7. The kerogen g value decreases from 2.0033 to 2.0022 and the linewidth increases from -7 to (2) Hwang, P.T.R.; Pusey, W. C., US.Patent 3740641, 1973. (3) Ishiwatari, R.;Ishiwatari, M.; Kaplan, I. R.; Rohrbach, B. G. Nuture (London) 1976,264, 247-9.
0887-0624,I87,12501-0501$01.50 . . . .., I O. 0 1987 American Chemical Society ~
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502 Energy & Fuels, Vol. 1, No. 6,1987
-8 G with increasing degree of alteration. The trends are consistent with previous but the low g values observed after treatment are not intrinsic to the organic material. Eaton et ala4obtained ESR spectra for a set of Wyoming shales and a Green River oil shale (g = 2.0034), for a Fischer assay spent shale, and for the shale oil produced during the assay (g = 2.0031). They found that the integrated intensity of the organic free radical signal in the raw shale correlates roughly with the Fischer assay oil yield, suggesting that the organic matter is uniform and that the ESR intensity is proportional to the total organic carbon level in the system. They later studied Q-band (35.2 GHz) ESR on these samples and found that the free-radical signal is a composite spectrum due to the superposition of several types of radical species with different g value^.^ Harrel16determined spin concentrations for raw, beneficiated, and spent Eastern (Alabama) and Western (Wyoming) shales. The ESR spectra of the more aromatic and highly phenolic Eastern shales were more intense and complex than those of Western shales. When raw Eastern shales are decomposed, the spin concentration generally decreased as the Fischer assay yield increased. These results differ from Marchand's' and the linear relationship that Eaton and co-workers4observed. This apparent inconsistency can be resolved if one considers that the free radicals in the oil shale are primarily on the aromatic molecules and that the oil yield arises primarily from the aliphatic fraction. Aizenshtat et aL7 studied ESR parameter changes from whole shales and kerogens at several stages of stepwise pyrolysis (ambient to 700 O C at 50 OC increments). Kerogen samples provided information on organic matter type and extent of conversion, while spectra on the whole shales were complicated by the presence of paramgnetic impurities. The present work focuses on the initial stages of kerogen pyrolysis. We have therefore chosen the range from 350 to 375 "C for the present experiments, since conversion of the organic matter occurs on a time scale of hours at these temperatures. Furthermore, many of the lighter pyrolysis products that appear during higher temperature pyrolyses occur as a result of secondary reactions, while the initial dealkylation reactions may yield relatively high molecular weight alkyl materials. Thus, an examination of the volatile component alone does not provide an adequate measure of the conversion that occurs. For this reason we have also used solvent extraction on the thermally treated shales to remove these heavy, yet labile, organics from the shale. This procedure is justified because the starting shales contain relatively small amounts ( .E
qli: 0
2
m 8
2.I
0-i
o
;
n x
x
Shale/% mp X GROS 350'C
LI
W CROS 57S.C
B
0 ROSSSD'C
m
ROS s 7 9 c I
Time (Hours) Figure 5. Rise of radical densities during thermal treatment.
served width. In shales there is also the possibility of broadening of the ESR absorption due to magnetic mineral species. Mineral effects can contribute to broadenings on the order of several gauss, as the demineralization studies above indicate. Therefore, line width variations with kerogen treatment are not easily predictable for the whole shale samples. ESR Carbon Radical Densities. Since extensive bond-breaking is occurring during the dealkylation process, it is not surprising that the carbon radical density increases after thermal treatment. The increase in carbon radical density with treatment time at 350 and 375 OC is shown in Figure 5 for the thermally treated shales. The radical density increases in the Green River oil shales are relatively small at both temperatures, in spite of the fact that there is a high degree of organic conversion. In contrast, the radical density increase in Rundle shale samples is much stronger, exceeding a factor of 5 for 6-h treatments at 375 OC. The number of stable radicals remaining behind in the sample after thermal treatment will depend on several factors: the number of reactions that occur during the treatment, the ability of the remaining molecules to form stable radicals, and the presence of reactive species from either the products or the environment that may react with the radical. Simple considerationssuggest that the number of primary reactions that result in a remaining stable radical are relatively small, perhaps 1 in 100. The difference in response between the Green River and Rundle shales could reflect the presence of larger aromatic species in the Rundle shale (radicals tend to be more stable on larger aromatic molecules because of the greater delocalization of the unpaired electron's wave function) or because of more effective radical quenching by products or the environment. Effects of Soxhlet Extraction. Examination of the residues of the Soxhlet extraction process allows us to understand the changes in ESR properties that occur in the presence of a solvent. Comparisons are given for Green River and Rundle shales in Table 11. In most of the Green River shale samples, there is a tendency for the g value to increase in the sample after Soxhlet extraction, which may reflect some oxidation of the organic during or after the extraction process. The line widths fall with extraction for Green River samples, while they remain nearly constant in the Rundle samples. One particularly interesting comparison is in the change in radical density before and after the extraction. As mentioned in the previous discussion, one normally tracks the number of spins per gram of carbon in the sample. However, in the present case, such an analysis might be misleading. The most favorable sites for the carbon radicals should be in the residue component of the system, with a relatively small number occurring in the more aliphatic extractables. Thus an increase in the radical
Table 11. ESR Properties for Thermally Treated and Extracted Samples" radial density w t 70 H~ 1o17jg iols/g io18/g sample of C g value of sample of C of CI GROS 350 OC 16.5 2.00327 7.89 1.00 0.60 0.60 start 1.41 16.1 2.00314 8.13 1-h heat 0.93 0.85 14.2 2.00327 7.86 0.94 1-h extr 0.66 0.57 1.5-h heat 14.96 2.00290 8.37 0.95 0.64 0.57 12.09 0.00321 7.30 1.58 1.5-h extr 1.31 0.95 14.24 2.00302 8.25 2-h heat 1.46 1.02 0.95 2-h extr 12.28 2.00312 7.54 1.84 1.50 1.11 GROS 375 "C 1-h heat 1-h extr 2-h heat 2-h extr 3-h heat 3-h extr
2.00308 2.00312 2.00299 2.00313 11.22 2.00288 5.20 2.00307
7.30 7.03 7.06 6.90 7.18 6.79
1.54 2.09 1.27 3.39 1.95 1.50
1.13 2.33 1.05 4.85 1.74 2.89
0.93 1.26 0.77 2.09 1.18 0.89
Rundle 350 "C start 1-h heat 1-h extr 1.5-h heat 1.5-h extr 3-h heat 3-h extr
13.19 2.00409 6.64 13.31 2.00381 8.84 12.12 2.00354 8.72 13.33 2.00352 8.72 12.67 2.00366 8.59 12.87 2.00346 8.37 12.13 2.00351 8.47
0.95 1.98 1.72 2.01 1.70 2.59 2.47
0.77 1.45 1.42 1.45 1.34 1.96 2.04
0.77 1.46 1.31 1.48 1.29 1.92 1.87
Rundle 375 "C 1-h heat 1-h extr 2-h heat 2-h extr 3-h heat 3-h extr 6-h heat 6-h extr
12.37 11.14 11.61 9.81 11.07 8.61 10.02 6.42
2.48 3.15 3.07 3.36 4.37 3.34 2.86 2.86
1.95 2.83 2.57 3.43 3.85 3.88 5.50 4.46
1.83 2.38 2.26 2.53 3.23 2.52 4.06 2.18
13.68 8.96 12.08 7.04
2.00333 2.00312 2.00324 2.00321 2.00324 2.00325 2.00279 2.00296
8.13 8.23 7.98 7.98 7.76 8.25 8.37 8.37
"The radical densities are defined in the text; CI = C initial.
density might well be observed in a case where no net radicals have been created, since the net amount of organic will diminish with extraction. To test for such effects, both the number of spins per gram of carbon and the number of spins per gram of initial carbon are included in Table 11. For most of the 350 O C treatments, the number of spins per gram of initial carbon is approximately the same before and after the extraction process (the only exception being the 1-h Green River shale treatment). This suggests that the radicals do reside in the residue and that their number is not significantly affected by extraction. By contrast, for the 375 "C samples the number of spins per gram of initial carbon falls upon extraction. This could arise either from removal of some of the radicals from the extracted fraction or quenching of the radicals during the extraction process. Kerogen Concentrate Pyrolysis. In order to distinguish thermal and mineral-related conversion effects, similar thermal treatments were performed on kerogen concentrates of both shales. Table I11 shows that the variations are very similar to those seen for the thermally treated whole shales. The g values fall to around 2.0028, the line widths fall, and the radical densities increase. Interestingly, the radical density increase for both kerogens is substantially greater, per gram of carbon, than it was for the whole shale. The effect is particularly obvious for the Green River shale case. Property Correlations. As these discussions indicate, the ESR properties of these materials are not entirely
ESR Studies of Kerogen Conversion
Energy & Fuels, Vol. 1, No. 6,1987 505 0.6,
Table 111. Carbon Radical Behavior for Kerogen Concentrates (375 OC Treatments) sample/ treatment"
wt %
of C
H/C
g value
width, G
0
1018 spins/g of c
0
Temp
-
1
0
.
350C
B
0 375c
,
3
5
7
A C radical (t) / C radical (stort) Figure 7. Increase in radical density with carbon loss for Rundle shales.
450-
1400-
I
s I
0
ISO-
0
7
drop in g value and line width that accompanies the increase in radical density for the kerogen concentrates (Table 111) could also be explained on that basis. Finally, the radical density increase should correlate with the loss of carbon during the pyrolysis if the radical density destruction mechanisms outlined in the last section are not operative. Clear-cut evidence for such an effect is not seen for the Green River oil shales, but a striking increase is observed for the Rundle samples (Figure 7).
. ' .. . .
5 300-
>I
*
B
250-1
Conclusions In this study of the pyrolysis properties of Green River and Rundle oil shales, the number and type of carbon radicals are observed to change with increasingly severe thermal treatment. Generally, g values and line widths fall while radical densities increase, although the comlex nature of the organic-mineral mixture of the shale requires some careful analysis of the data. Comparison with similar treatments on the kerogen concentrates enables us to identify labile radical forms in the Rundle shale and indicates that the increase in radical density is considerably higher in the absence of the mineral matter. Experimental Section General Procedure for Thermal Heat Soak and Extraction of Raw Oil Shales. The heat soak portion of the conversion was carried out in a minipyrolyzer. The oil shale sample (- 1-3 g) was weighed and placed into a tared quartz reaction tube. Onto the top of this reactor tube was clamped a tared condenser tube. The condenser consisted of a Pyrex tube with a ground-glass fitting, packed with tared 3-mm Pyrex beads that were supported by a stainless-steel screen and quartz wool at the bottom and glass wool a t the top. A stainless-steel tube (l/g in. 0.d.) through the condenser packing and halfway into the reactor tube was used to provide a 40 cm3/min nitrogen purge to help sweep out primary pyrolysis products to the condenser section. The condenser section was packed in dry ice and allowed to cool for approximately 5 min with the nitrogen purge on. The entire assembly was then placed in a heating block at a given temperature, 350 or 375 "C. The heating block was simply a steel cylinder with a 1in. diameter hole drilled out of the middle, but not all the way through. This cavity was partially filled with gallium (mp 29.8 "C, bp 2403 "C) to facilitate rapid heat up (
