Energy & Fuels 1991,5, 860-866
860
Table VII. ComDonents of Blends for Viscosity Measurements blend series high-viscosity component low-viscosity component 96.5% RAL2 + 3.5% HGAL 97.5% RZ2 + 3.5% HGZ 1 2 100% RILl 100% HGAL 3 100% RILl 96% RZ2 + 4% HGZ 4 100% RILl 100% HGZ 97% RAL2 + 3% HGAL 94.5% RZ2 + 5.5% HGZ 5 94.5% RZ2 + 5.5% HGZ 98% RILl + 2% HGAL 6 96.5% RAL2 + 3.5% HGAL 88% RZ2 + 12% HGZ 7 87% RZ1+ 13% HGZ 96% RILl + 3% HGAL 8 60% RZ1+ 40% HGZ 100% RILl 9 74% RZ1+ 26% HGZ 97% RIL2 + 3% HGIL 10 66% RZ2 + 34% HGZ 11 96% RIL2 + 4% HGIL 66% RZ2 + 34% HGZ 12 99% RILl + 1% HGAL 85% RZ2 + 15% HGZ 97% RILl + 3% HGAL 13 100% HGAL 14 94.5% RZ2 + 5.5% HGZ 100% HGAL 99% RAL2 + 1% HGZ 15 80% RZ1+ 20% HGZ 99% RAL2 + 1% HGAL 16 98% RZ2 + 2% HGIL 90% RZ1+ 10% HGZ 17 100% HGAL 18 98% RZ2 + 2% HGIL 19 97% RAL2 + 2% HGAL 100% HGAL 97% RALS + 2% HGAL 86% RZ2 + 14% HGZ 20 21 100% RS 100% HGZ 22 100% RS 100% RALl 48% RALl + 52% HGIL 23 100%RS 85% RALl + 15% HGIL 24 100% RS
i in the mixture, by volume. Since the refineries prefer the tabulated blending indices for predicting the blended viscosity, it was decided to attempt to express mathematically the relation between
VBI and kinematic viscosity. The following expression was found to give very good results VBI = e x p ( 4 . 1 7 9 7 5 7 ~ ~ ~ ~ " " ~ ~ ) (13) where VBI is the viscosity blending index, by weight, and u the kinematic viscosity (cSt). The correlation coefficient with actual blending indices is 0.999, and a graphic representation of the function is shown in Figure 7. All of our results in the viscosity blending experiments are presented in Table VI, whereas Figure 8 is a schematic representation of actual and calculated viscosity values for two of the blend series. Finally, Figure 9 shows a comparison of all 264 of the measured viscosity values with the values calculated from relations 8 and 13. Conclusions Mathematical expressions are presented for some nonadditive properties for mixing gas oil and residual fuel fractions. Excellent correlations are obtained for the blended values of the pour point, the flash point, and viscosity, thus obviating the need for tabulated values of the blending indices, which are currently used in industry. Acknowledgment. We thank Petrola Refinery, Elefsis, Greece, for supplying us with most of the crude oil components that were used in this study.
Behavior of Oxidized Type I1 Kerogen during Artificial Maturation Patrick Landais,*ft Raymond Michels,t Jacky Kister,* Jean-Marie Derepge,s and Zouhir Benkheddat CREGU and GDR CNRS-CREGU, BP 23,54501 Vandoeuure-16s-Nancy Cedex, France, Centre de Spectroscopie Mole'culaire, Faculte' des Sciences et Techniques St. Je'rGme, 13397 Marseille Cedex, France, and CRIS, Universitd de Louuain-la-Neuue, 1 Place Louis Pasteur, 1348 Louvain-la-Neuve, Belgium Received May 10, 1991. Revised Manuscript Received July 5, 1991 A series of artificially oxidized type I1 kerogens (ventilated oven, 140 "C, 8-256 h) from the Paris Basin (France) has been pyrolyzed in cold-seal autoclaves at temperatures ranging between 250 and 450 OC for 24 h at 100 MPa. Oxidates and pyrolysates have been characterized by I3C solid-state NMR, FTIR spectroscopy, Rock-Eva1 pyrolysis, elemental analysis, and CHC13extraction. Results indicate that oxidation is responsible for an important decrease of the petroleum potential and for the increase of different oxygen-bearingfunctions' (carbonyl, carboxyl, esters, ethers) concentration. The behavior of the different oxidates during artificial maturation is characterized by two distinct stages: oxygen removal (250-300 "C)and hydrocarbon production (300-450 "C). It is shown that oxidation induces an important decrease of the hydrocarbon yield during maturation and that no regeneration of petroleum potential can be observed. Comparison of the most oxidized kerogen (256 h) with an unoxidized type I11 coal of similar initial elemental composition has also been carried out.
Introduction The effects of oxidation on organic matter composition have been widely studied in the fields of coal processing, soil sciences, or petroleum geochemistry. Increases of alkali CREGU and GDR CNRS-CREGU.
* Facult6 des Sciences et Techniques St. JCrBme. 1 Universitg
de Louvain-la-Neuve.
solubility' and aromaticity,2decrease of coking quality? and modifications of optical properties as well as pyrolysis products distribution4 have been quoted among the main (1)Jensen, E. J.; Melnyk, N.; Wood,J. C.; Berkowitz N. Ado. Chem. Ser. 1964,55, 621-642. (2) Dereppe, J. M.; Moreaux, C.; Landais, P.; Monthioux, M. Fuel
1987,67, 764-770. (3) Seki, H.; Ito, 0.; Lino, M. Fuel 1990, 69, 1047-1051.
0887-0624/91/2505-0860$02.50/00 1991 American Chemical Society
Energy & Fuels, Vol. 5, No. 6, 1991 861
Behavior of Oxidized Type 11 Kerogen consequences of organic matter oxidation. Most of these compositional and structural changes are related to the depletion of aliphatic C-H moieties and the subsequent increase of oxygen functional groups concentration."8 Such chemical modifications are of scientific importance when we consider petroleum source rocks as far as degradative processes associated with organic matter oxidation are responsible for a drastic loss of hydrocarbon potential.2sBJ0 Weathering and surface oxidation result in structural and functional transformations which mainly affect the part of the kerogen which preferentially cracks during thermal maturation to produce mobile and hydrogen-rich Various pyrolysis experiments have demonstrated that the yield of hydrocarbons or tars generated upon heat treatment decreases with increasing oxidati~n.~J'J~ Similar conclusions have been deduced from the comparison of the oil potential of organic matter of different origin which decreases from type I (algal) to type I1 (planktonic) and type I11 (humic) as a function of their respective hydrogen and oxygen contents.16 Then, it was interesting to investigate the behavior of a series of oxidized kerogens during thermal maturation in order to check the effects of oxidation on their ability to generate hydrocarbons. As far as natural systems cannot provide complete series of maturation of oxidized kerogens and as secondary phenomena (hydrocarbon migration, biodegradation) can interfere with the oxidation-maturation pair, laboratory experiments have been performed in order to obtain both the oxidates and the pyrolysates series. It has been shown that oxidation experiments imperfectly simulate natural oxidation and weathering mainly because of the high temperatures used.17 However, fundamental changes in chemical composition, i.e., oxygen fixation and hydrogen consumption, are roughly similar in natural and artificial oxidation. On the other hand, artificial maturation performed in a confined system is able to reproduce most of the transformations undergone by a type I1 kerogen during natural thermal maturation.18 Thus, one can expect from laboratory experiments to establish the principal changes that may occur during the maturation of oxidized kerogens. The objective of this work is to document the effects of thermal treatment on a series of artificially oxidized type (4)Furimsky, E.; MacPhee, J. A.; Vancea, L.; Ciavaglia,L. A,; Nandi, B. N. Fuel 1983,62,395-400. (5)Fredericks, P. M.; Warbrooke, P.; Wilson, M. A. Om. Geochem. 1983,5, 89-97. (6)Calemma, V.;Rausa, R.; Margarit, R.; Girardi, E. Fuel 1988,67, 764-770. (7)Banerjee, A. K.;Choudhury, D.; Choudhury S. S. Fuel 1989,68, 1129-1133. (8) Rausa, R.; Calemma, V.; Ghelli, S.; Girardi, C. Fuel 1989, 68, 1168-1172. - - -.- - (9) Espitalig, J.; Deroo, G.; Marquis, F. Rev. Inst. Fr. Pet. 1986,6, 755-784. (10)Landais, P.; Monthioux, M.; Meunier, J. D. Org. Geochem. 1984, 7,249-260. (11)Nehn, C. R. In Chemiatry of Coal Weathering;Coal Science and Technology 14;Nelson, C. R., Ed.; Elsevier: New York, 1989 pp 1-32. (12)Nipaise, G.Rapp. Inst. Fr. Pet. 1977,21384. (13)Ingram, G. R.; Rimstidt, J. D. Fuel 1984,63,292-296. (14)Gavalas, G. R. In Coal Pyrolysis; Coal Science and Technology 4;Gavdaa, G. R. Ed.; Elsevier: New York, 1982;168 pp. (15)Solomon,P. R.; Serio, M. D.; Despande, G. V.; Kroo, E. Energy Fuels 1990,I, 42-53. (16)Tissot, B. P.; Pelet, R.; Ungerer, Ph. Am. Ass. Pet. Geol. Bull. 1987,71, 1445-1466. (17)Huggins, F. E.; Huffman, G. P. In Chemistry of Coal Weathering; Coal Science and Technology 14;Nelson, C. R., Ed.; Elsevier: New York, 1989;pp 33-60. (18)Landais, P.; Michels, R.; Poty, B. J. Anal. Appl. Pyrol. 1989,16, 103-115.
Oh
' e
4 8 h
t
32 h
A e64h
e
Artificial
128 h
e256h V,'
I
0.0
or1
0,z
03
0,4
O K at.
Figure 1. Evolution of the elemental composition of the Paris Basin type I1 kerogen during artificial oxidation. Comparison with natural oxidation of a similar kerogen.12
I1 kerogens and to compare their behavior with that of a type I11 coal of similar composition during artificial maturation. Emphasis will be put on the evolution of the petroleum potential during oxidation and subsequent maturation and the implications on natural systems.
Experimental Section S t a r t i n g Material. Experiments were performed on a type I1 kerogen from the Toarcian of the Paris Basin (France). Raw kerogen was concentrated by HCl/HF acid treatment following the method developed by Durand and Nicaisel8 and showed geochemical characteristics compatible with the end of the diagenetic stage (hydrogen index (HI) = 483 mg/g; H/C = 1.28; O/C = 0.074). Oxidation. Three grams of type I1 kerogen was ground and passed through a 50-pm sieve and then heated to 140 "C in a ventilated oven for times increasing from 8 to 256 h (8, 32, 64, 128,256 h) in order to obtain a representative series of oxidation. Artificial Maturation. Raw kerogen and 32, 128, and 256 h oxidates were artificially matured in a confined-pyrolysis system described in detail elsewhere.18 About 150 mg of dried and powdered sample was placed under argon atmosphere inside a 50 mm long sealed gold tube. The gold tube was then introduced in a cold-seal autoclave. Standard conditions of heat treatment used were 24 h isothermal stage and temperatures of 250, 300, 350,400, and 450 "C. A hydrostatic external pressure of 100 MPa was exerted on the gold tube in order to ensure a strong confinement. Confined pyrolysis is able to reproduce most of the transformations undergone by a type I1 kerogen during natural maturation." Several discrepancies between natural and artificial maturation have been noticed on type 111coals. They have been related to the presence of free but trapped hydrocarbons in naturally matured coals and mainly concern the evolution of the chloroform extraction rate.21 l3C C P / M A S S o l i d - s t a t e NMR. Only the oxidates were characterized by 13CNMR on a Bruker CXP 100 spectrometer. Partial polarization experiments were conducted with cross polarization times ranging from 0.5 to 10 ms and led to choice of a standard cross polarization of 1ms. NMR spectra were divided into 10 bands labeled 01 and 0 2 for C-0 and C 4 carbons (220-164 ppm), A 1 to A4 for aromatic carbons (164-104 ppm), and S1 to S4 for saturated carbons (104-0 ppm). The precise assignment and the boundaries between the different bands were determined from the work of different authors referenced in Dereppe et al.= In this study, the various NMR parameters will be discussed on a comparative standpoint. FT-IR. FT-IR spectra were obtained on a Fourier transform Nicolet 20 SXB spectrometer with a spectral resolution of 4 cm-'. (19)Durand, B.;Niqaise, G. In Kerogen; Durand, B., Ed.; Technip: Paris, 1980;pp 35-54. (20)Landais, P.; Monin, J. C.; Monthioux, M.; Poty, B.; Zaugg, P. C. R. Acad. Sci. Paris 1989,308 (ll),1161-1166. (21)Monthioux, M.; Landais, P. Fuel 1987,66,1703-1708. (22)Kister, J.; Guiliano. M.: Mille. G.: Dou. H. Fuel 1988, 67, 1076-1082.
Landais et al.
862 Energy & Fuels, Vol. 5, No. 6, 1991 Table I. Geochemical Characteristics of Raw Type I1 Kerogen and Osidates" HI, 01, T,,, COO, samples H/C O/C O/H mg/g mg/g "C % raw sample
BP
oxidates BP 8 h BP 32 h BP 64 h BP 128 h BP 256 h
1.28
0.074
0.058
483
11.7
431
8.2
1.27 1.21 1.14 1.04 0.99
0.152 0.18 0.222 0.293 0.346
0.119 0.149 0.195 0.283 0.348
464 436 394 214 220
12.4 42.4 62.1 91.4 119.5
420 422 420 424 421
15.5 17.9 21.3 26.4 29.9
"HI= Rock-Eva1 hydrogen index. OH, Rock-Eva1 oxygen index. peak temperature of hydrocarbon production during Rock-Eva1 pyrolysis. COO, corrected organic oxygen (see text). ?'-,
?b2D
363:
3
k
2503
2400
2noo
l*!2
WCrEhUMRCR
ILW.
130
:-1
Figure 2. FTIR spectra of the different oxidates.
Table 11. Spectroscopic Characteristics of the Raw Type I1 Kerogen and Osidates' F,, AI, A3/xA, 01 + 0 2 , CH3/ samples 70 % CHJCH, % % CH,+CH, raw sample ~
BP
28.6 4.2
0.39
20.2
0.6
0.65
oxidates 1.2 0.67 0.43 20.3 BP 8 h 33.8 4.4 BP 32 h 36 5.3 0.45 20.2 1.3 0.69 BP 64 h 37.8 5.6 0.47 20 2.2 0.79 20.1 5.2 0.89 BP 128 h 41.6 7 0.51 BP 256 h 47.3 8.2 0.59 19.9 7.2 0.95 "F, 13C NMR aromaticity factor; AI, percentage of orygen-substi-
tuted aromatic carbons (I3C NMR); CHa(CH2, '*C NMR methyl/ methylene ratio. A3/xA, contribution of bridgehead aromatic carbons in total aromatic carbons (13CNMR); 01 + 02, percentage of carbonyl carboxyl carbons (I3C NMR); CH2/CH2 + CH3, 1370/1460 cm-' infrared band area ratio.
+
KBr pellets of the chloroform-extracted raw kerogen, oxidates, and pyrolysates were prepared as described by Kister et using a 1:150 kerogen to KBr ratio. The precise assignment of the and different IR absorbance bands are given in Kister et Guiliano et alea Kerogen Characterization. Chloroform-extractedraw kerogen, oxidates, and pyrolysates were analyzed by Rock-Eval pyrolysis and C, H, and 0 elemental analysis. Hydrogen index (HI = amount of hydrocarbonsreleased during pyrolysis in milligrams per gram of organic carbon),oxygen index (01 = amount of C02 released during pyrolysis in mg of C02 per g of organic carbon), and T,, (peak temperature of hydrocarbon production in "C) were deduced from Rock-Eva1pyrograms. Corrected organic oxygen (%COO = %O/(%C + %H + %O)u and corrected orwere chosen ganic carbon (%COC = % C / ( S C + %H + as common oxidation and maturation parameters, respectively. Results a n d Discussion Characterization of the Oxidates. Elemental composition of the different oxidates have been plotted in a van Krevelen diagram (Figure 1). Oxidation is characterized by an increase of the O/C atomic ratio and a decrease of the H/C atomic ratio (Table I). It is worth noticing that the trend of artificial oxidation is very similar to that deduced from the analysis of a natural surface series of oxidation of a Paris Basin type I1 kerogen.12 The end member of the oxidation series falls in the range of the type I11 coals. Others parameters such as the Rock-Eval oxygen index (01)(Table I), the intensity of the I3C NMR bands associated with oxygen-bearing carbons (oxygen-substituted aromatic C, A1 band; carbonyls and carboxyls, 01 + 0 2 band) gradually increases with oxidation time and COO (23) Guiliano, M.; Mille, G.; Kister, J.; Muller, J. F. J . Chim. Phys.
1988,85, 963-969.
(24) Michels, R. Diplome $Etudes Approfondies, I.N.P.L., Nancy, 1990; p 65.
0
10
20
30
coo (%) Figure 3. Comparative evolution of the petroleum potential (HI in mg of hydrocarbons per g of TOC) and of the percentage of saturated carbons determined by I3C NMR during oxidation. COO: corrected organic oxygen. (Table I, Table 11). Complementary information on oxygen fixation can be derived from FTIR spectra (Figure 2) which show the following features: (a) increase of the 1700-cm-' band assigned to carbonyl and carboxyl functions; (b) shift of the maximum of the 1700-cm-l band from 1695 cm-I (raw kerogen) to 1715 cm-' (256 h oxidate); (c) after 32 h, appearance of a shoulder located a t 1765 cm-' typical of aryl ester functions22which increases with oxidation time; and (d) increase of the 1100-1300-~m-~ broad band associated with various oxygen-bearing functions; the gradual shift of this band towards higher wavenumbers could be explained by the predominance of aromatic ethers in the most oxidized sample.2 Similarly, parameters reflecting the hydrogen content of the kerogen, namely (a) the 2800-3000-cm-' IR band attributed to aliphatic CH2 and CH3 (Figure 2): (b) the saturated carbons I3C NMR bands (Figure 3 ); and (c) the Rock-Eva1 hydrogen index (HI) (Figure 3, Table I), decrease with oxidation. Furthermore, the average length of aliphatic chains determined either by 13CNMR (methyl/methylene ratio, Table 11) or FTIR (CH3/CH2 + CH3 = 1370/1460 cm-' band ratio) drastically decreases with oxidation. The lack of agreement with the findings of Fredericks et al.6 which indicated that there was no preferential oxidation among the aliphatic groups of coal can be related with the high initial aliphaticity of the raw kerogen selected for this study. Conversely the 13C NMR percentage of aromatic carbons increase with oxidation from 28 to 47% (Table 11). However, this evolution does not correspond to an aromatization and must be interpreted as the preferential removal of saturated carbons as far as neither the bridgehead aromatic carbon/total aromatic carbon ratio (A3/CA, Table 11) determined by 13C NMR nor the position of the 1600-cm-' IR band shows any significant variation with oxidation. Surprisingly, the Rock-Eval 7'is lower for all oxidates than for raw kerogen whereas the previous studies on natural and artificial oxidation of type
Behavior of Oxidized Type II Kerogen
Energy & Fuels, Vol. 5, No. 6,1991 863
Table 111. Geochemical Characteristics of the Pyrolysates' - COC, HI, 01, WL, OE/TOC, T-9
HIC
sample 250/R 250j32 2501128 2501256 3001R 300/32 3001128 3001256 350/R 350/32 3501128 3501256 400/R 400/32 400/128 4001256 450/R 450/32 4501128 4501256
OIC 0.054 0.124 0.148 ,O.l6l 0.041 0.049 0.078 0.091 0.043 0.041 0.052 0.065 0.025 0.028 0.041 0.05 0.026 0.021 0.038 0.027
1.18 1.16 0.94 0.94 1.08 1.04 0.86 0.81 0.65 0.62 0.63 0.71 0.54 0.51 0.53 0.5 0.48 0.47 0.41 0.43
%
mg/g 477 359 227 184 318 274 173 122 52 55 44 81 18 21 9 9 7 9 5 4
85.5 79.2 78.3 77.3 87.3 86.8 85 84.1 89.9 90.4 89.1 87.3 92.8 92.6 91.1 90.2 93 93.7 92.2 93.4
mg/g
8.3 9.3 15.3 17.7 1.6 3.1 7.6 7.2 3.3 4.3 5.8 6.2 2.4 3.8 4.5 3.5 4.2 4.6 5 3
%
O C
432 430 429 431 441 439 441 445 450 456 462 470
mg/g 51.4 9.5 2.3 1 327 155 26.7 35 333 232 91.5 64 93.6 43 31.9 12.8 29.9 31.4 6.2 2
1.73 2.44 7.2 7.7 2.93 6.12 10.2 13.25 8.81 12.11 17.4 17.75 18.75 20.34 24 24.4 30.06 30.6 30.1 29.31
545 546
CH3/CH2+ CH3
P1600
0.778 0.813 0.944 0.869 0.846 0.833 0.97 0.91 1.05 0.947 0.969 0.944 1.04 1.03 1.045 1.02 1.11 1.1 1.095
cm-l
1620 1636 1622 1621 1595 1590 1592 1592 1595 1597 1592 1590 1593 1592 1588 1588 1572 1587 1578 1578
1.11
'250, pyrolysis temperature ("C);WL, weight loss; OE/TOC, CHCl, extractltotal organic carbon; P1600 cm-', position of the 1600-cm-* infrared band. See Tables I and I1 for other abbreviations. 0 32hours
128hours
"
0,o
0.1
02
03
0,4
OIC at.
Figure 4. Evolution of the different oxidates and raw kerogen during artificial maturation plotted in a H/Cvs O/C diagram. Pyrolysis temperature, 250 "C. I11 coal have shown an increase of . , T with oxidation.1o* Once again, this may be related to the high initial aliphaticity of the raw kerogen and the absence of a real aromatization process during oxidation. Furthermore, the increase in T,, during oxidation of type I11 coals has been associated with the higher thermostability of the oxidates due to oxygen bridges between aromatic structures. Because of the low aromaticity of the raw kerogen selected for this study (13C NMR Fa = 28.6%; Table 11) such a process may not be enhanced. The artificial oxidation of the Paris Basin type I1 kerogen is then characterized by a general increase of the oxygen content (mainly carboxyl and carbonyl functions), a drastic decrease of the petroleum potential, and the absence of significant variations of the aromatization and thermostability indicators. Lastly, the characteristics of the end member of the oxidation series are roughly similar to that of a type I11 coal of similar elemental composition. Behavior of Oxidates during Artificial Maturation. The evolution of organic matter during maturation can be characterized by two main processes (i) the removal of the oxygen-bearing functions and (ii) the decrease of the petroleum potential and the genesis of hydrocarbons. Geochemical data deduced from the analysis of the different pyrolysates are reported on Table 111. As in natural and artificial maturation of unoxidized material, oxygen release (25) Williams, D. H.; Flemming, I. Spectroscopic Methods in Organic Chemistry; McGraw-Hill: London, 1966; 222 pp. (26) Landais, P. Org. Geochem., in press.
I
200
300
400
500
Pyrolysis tempenhue PCI
Figure 5. Weight loss evolution during the pyrolysis of the different oxidates and raw kerogen. occurs before the beginning of oil genesis. The H/C vs O/C diagram of Figure 4 clearly evidences these two main phases of thermal maturation. The more oxidized the starting material is, the higher the O/C ratio decrease noticed between 0 and 300 "C. Furthermore, it is interesting to note that, whatever the starting material, the end of the main phase of oxygen release takes place for a pyrolysis temperature of 300 "C. The Rock-Eval oxygen index (01)shows a quite different evolution since a t the 250 "C pyrolysis step; all oxidates present a very low 01 value (Table 111). This probably signifies that only part of the oxygen is recorded on S3 peak of the Rock-Eval pyrogram and that the less labile oxygen-bearing functions eliminated during higher temperature pyrolysis stages are not taken in account in the S3 peak. Infrared spectra confirm that the main phase of oxygen elimination occurs during the 250 "C pyrolysis run. Esters (1765 cm-9, and carbonyl + carboxyl functions are totally removed at 250 and 300 "C, respectively, except for the 256-h oxidate for which carboxyl functions are observable until 350 "C. The intensity of the broad band centered at 1200 cm-I and attributed to ethers bonds and various C-0 functions drastically decreases at 300 "C for the raw sample and 32-h oxidate. For the 128- and 256-h oxidates, this band progressively shifts toward higher wavenumbers, thus probably indicating the predominance of aryl-0-aryl and aryl-0-alkyl ethers in the most mature samples.22 Then, most of the oxygen-bearing functions are eliminated during the first stages of pyrolysis. However, for the most oxidized sample, carboxyl, ester, and ether groups remain
864 Energy & Fuels, Vol. 5, No. 6,1991
Landais et al. 4o 1
Raw (11)
32hours 8 128hours A 256hours 0
0
8
32 hours 128hours
A
.
0
20
40
,
.
60
,
.
,
100
80
.
i
120
0 1 (mgC02 /g TOC)
Figure 6. Plot of the Rock-Eva1 data of the different oxidates and raw kerogen. 01, oxygen index; HI, hydrogen index. 60-
50
ap
-
32Hours 128Hours A 256Houn 0
8
-
A
40
0
5
30: 2010
-
200
250
300
350
400
450
500
T ("C)
Figure 7. Variations of the petroleum potential loas with pyrolysis temperature. Hb, initial petroleum potential (mg of hydrocarbons per g of organic carbon); AHI, HI(T) - HI(T + 50 "C).
until the 350 "C pyrolysis step. This can be interpreted either by a slackening of the rate of cleavage of oxygenbearing functions with increasing oxygen content or, more probably, by the persistence of relatively thermally stable functions. The weight loss percentage after the gold tubes are pierced gradually increases with maturation from 2 to 7% (250 "C) to 30% (450 "C) (Figure 5). Raw sample weight loss evolution is exponential, the major increase occurring during the 400 and 450 "C pyrolysis stages and corresponding to the genesis of low molecular weight hydrocarbons during the end of the catagenetic stage.2o On the contrary, the evolution of the 256-h oxidate weight loss fits with a straight line. This probably indicates that the lower production of gaseous hydrocarbons (Cl-C6) is partly counterbalanced by the release of larger quantities of C02 and H20during the first steps of pyrolysis (250-300 The patterns of 32- and 128-h oxidates are intermediary and the weight loss percentage of the end members of the different maturation series is roughly similar (29.3-30.6%). Figure 6 indicates that, during the major stage of oxygen elimination (250 "C pyrolysis), the residual petroleum potential (Rock-Eva1hydrogen index, HI) only slightly decreases (between 16 and 18% of the initial value). The principal phase of hydrocarbon production occurs later, Le., during the 300 and 350 "C pyrolysis steps, thus confirming the observations made on the H/C-O/C diagram. Detailed inspection of the evolution of the hydrogen index (HI) during artificial maturation brings more information. The diagram of Figure 7 shows the evolution of the HI(T) - HI(T + 50 "C)/HI0 ratio versus pyrolysis temperature. It can be noticed that, for raw sample, 32 (27) Izuhara, H.; Tanibata, R.; Nishida, S. In Proceedings: 1985 International Conference on Coal Science, Sidney, 1985; Pergamon Press:
200
300
400
500
Pyrolysis TemperaturePC) Figure 8. Evolution of the CHClB extract yield with pyrolysis
temperature for a raw kerogen and oxidates.
h and 128-h oxidates, the principal phase of petroleum potential loss occurs during the 350 "C pyrolysis step. The 256-h oxidate behaves differently as far as the two peaks can be defined for the 300 and 400 "C pyrolysis steps, respectively. Such evolution can probably be related to the modification of the distribution of activation energies and has already been observed in the free-radical concentration of weathered coal during pyrolysis.m Especially, the oxidation phenomenon which provokes a drastic increase of the aromaticity factor and methyl/methylene ratio can be responsible for the modification of the rate of hydrocarbon release during maturation. It can also be suggested that this peculiar evolution corresponds to two distinct phases of oxygen elimination: 300 "C for labile oxygen-bearing functions and 400 "C for the more stable functions (ethers). Similarly, the percentage of CHC1, extractable compounds generated during maturation gradually decreases with the oxidation level. Figure 8 clearly evidences that oxidation is responsible for a sharp decrease of oil genesis during m a t ~ r a t i o n .Furthermore, ~ the maximum of oil production seems to be progressively shifted toward higher pyrolysis temperatures with increasing oxidation. This can be related to the higher thermostability of the oxidized samples which induces a delay in the cracking of the kerogen.29 As a matter of fact, the highest residual petroleum potential at 350 "C is that of the 256-h oxidate. Data on the solid residue also allow the comparison of the different oxidates behaviors during maturation. The infrared aliphaticity factor decreases with maturation for all oxidates. However, the main evolution occurs between 300 and 350 "C and is more pronounced for the raw sample. Conversely, the meyhyl/methylene + methyl ratio (1370/ 1438 cm-' IR bands ratio) gradually increases with maturation except for the 256-h oxidate for which a slight decrease at 250 "C can be noticed (Table 111). Aromatization of the pyrolysates can be depicted by the progressive shift of the 1600-cm-' IR band toward lower wavenumben (Table 111). The shift of this band from 1620 cm-I (aliphatic C=C) toward 1570 cm-' is observed in the spectra of the different maturation series. It can be related to the predominance of more conjugated C=C species in the more mature pyrolysates. Rock-Eva1 T,, is also known as a thermostability indicator which correlates with the aromaticity of organic matter.2 In this study, T,, increases with pyrolysis temperature for all oxidates and raw material (Table 111). However, until 350 "C, no significant variation can be associated with the oxidation level. A t 350 "C, T,, increases with the oxidation level from 450 to 470 OC. This should indicate that the effect of oxidation on the thermostability of the organic struc-
1985; pp 491-494.
(28) Bakr, M.; Yokono, T.; Sanada, Y. 1989 Int. Con/. Coal Sci., Tokyo, NED0 1989, 217-220.
(29) Davidson, R. M. Natural Oxidation of Coal; IEA Coal Research; 1990, IEACR/29, 76 p.
Behavior of Oxidized Type ZZ Kerogen
Energy & Fuels, Vol. 5, No. 6, 1991 865
51
200
300
400
500
Pyrolysis Temperature ("C)
Figure 9. Evolution of the aromatic 7-CH infrared band area with pyrolysis temperature for raw kerogen and oxidates.
Table IV. Comparison of the Geochemical Characteristics of Raw and Pyrolyzed Samples from an Unoxidized Mahakam Type I11 Coal and from the 256-h-Oxidized Type I1 Kerogen sample H/C O/C HI, mg/g 7'-, O C OE,mg/g Type 111 Coal raw 0.925 0.344 135 418 78 0.861 0.231 134 429 250 300 98.6 0.821 0.168 126 439 107 0.733 0.104 84 445 350 40 400 0.583 0.069 25 546 4 0.48 0.034 8 450
raw 250 300 350 400 450
0.995 0.939 0.813 0.705 0.497 0.427
Type 0.346 0.161 0.091 0.065 0.05 0.027
I1 256 h 220 184 122 82 9 4
421 431 445 463 546
1
34.5 tures can only be noticeable when the maturation stage 64 has reached the maximum of oil genesis. Such observation 12.8 for the 350 "C pyrolysates can be related with the con2 clusions previously drawn on the evolution of the residual petroleum potential. Aromatic CH infrared bands (720-920 cm-') of the different oxidates also show distinct evolutions during artificial maturation (Figure 9). Even if general trends are roughly similar-increase until 400 "C followed by a slight decrease at 450 "C-it must be pointed out that the integrated areas of the aromatic y C H bands of the 256-h pyrolysates always remain lower than for the other maturation series. Such behavior can be related to the higher initial oxygen substitution of the aromatic rings and to the 0,o 0,1 0.2 0,3 0,4 remaining oxygen bridges. It is worth noting that modiOK fication of aromatic CH pattern during oxidation is not Figure 10. Comparison of the respective behaviors during maalways easy to detect6J0and that artificial maturation can turation of the 256-h oxidate and an unoxidized type I11 coal of provide additional data for its interpretation. similar elemental composition plotted in a H/C vs O/C diagram. As previously noted in various pyrolysis studies of naturally or artificially oxidized coa1s,15*26w1 oxidation is responsible for an important decrease of the hydrocarbon yield.30 The preferential attack of aliphatic CH2 groups induces a drastic decrease of the average length of aliphatic chains and a significant loss of petroleum p ~ t e n t i a l . ~ ' . ~ ~ Consequently, the rate of hydrocarbon generation during confined pyrolysis is modified. Furthermore, no regeneration of petroleum potential due either to the prefere 50 ential removal of oxygen-bearing functions or to subsequent hydrogenation reactions during confined pyrolysis'* 0 60 70 80 90 100 can be expected. This suggests that petroleum source Coc (%) rocks submitted to weathering or oxidation will not genFigure 11. Evolution of the petroleum potential (HI) of the 256-h erate during thermal maturation the amounts of hydrooxidate and the unoxidized type III coal during maturation. COC: carbons which could be expected from the petroleum pocorrected organic carbon is chosen as a common maturity inditential of the unaltered kerogen. cator. Comparison with Unoxidized Type I11 Coal. In a van Krevelen diagram, the elemental composition of the ilarly during artificial maturation and that their evolu256-h oxidate falls in the range of the type I11 coals. Then, tionary paths remain parallel. However, during the diait was interesting to compare the behavior of this oxidized genetic stage, the rates of oxygen elimination are not simtype I1 kerogen with that of an unoxidized type I11 coal ilar; i.e., the 0.1 O/C ratio is reached at 300 "C for the type of similar O/C atomic ratio. An immature coal from the I1 oxidate and at 350 "C for the type I11 coal. This could Mahakam delta was selected for this comparison. The be explained by differences in the initial repartition of the Mahakam coal was pyrolyzed under the same conditions oxygen in the starting materials. 13C NMR data are in as the 256-h oxidate. The geochemical characteristics of agreement with elemental composition and indicate that the starting material and pyrolysates are compared in the percentage of carbons bonded to an oxygen are similar: Table IV. 17.5% for type I1 oxidate vs 17.9% for type I11 coal. Plot of the elemental compositions on a H/C vs O/C Nevertheless, the type of oxygen substitution is different, diagram (Figure 10) shows that both samples behave sim01 + 0 2 NMR bands (carbonyl, carboxyl, ketones, and esters) representing 41% of the total 0-bonded carbons in the type I1 oxidate and only 31% in the type I11 c0al.2~ (30) Jakab, E.; Windig, W.; Meuzelarr, H. L. C. Energy Fuels 1987, On the other hand, 0-substituted aliphatic C in ethers, 1, 161-167. (31) Joseph, J. T.: Mahaian, 0. P. Prepr. Pap.-Am. Chem. SOC.,Diu. alcohols, and methoxy groups are much more abundant Fuel Chem.-1989,34,931-945. in the type I11 coal (28% vs 12%). As far as low energies (32) Saxby,.J. D.; Lambert, D. E.; Riley, K. W. Fuel 1987,66,365-368. are required to remove carbonylic or carboxylic oxygen,14 (33) Landme. P.: Derepw, J. M.: Monthiour, M. C. R. Acad. Sci. Park 1988,306 ( l l ) ,1093-109?. one can expect the type I1 oxidate to be more quickly
7
Energy & Fuels 1991,5,866-868
866
70
80
90
100
coc (%) Figure 12. Comparison of the CHCl:,extract yield of the 256-h oxidate and type I11 coal during maturation. COC: corrected organic carbon is chosen as a common maturity indicator.
deoxygenated than the type I11 coal. Other discrepancies between the respective behaviors of the two starting materials during maturation can be noticed. Figures 11and 12 respectively plot the evolutions of HI and chloroform extract versus COC content. The petroleum potential (HI) of the type I1 oxidate remains higher than that of the type I11 coal during all the artificial maturation process (Figure 11)while the chloroform extract yield is generally lower. This could be explained by the initial length of the aliphatic chains of the two types of organic matter: oxidation is responsible for a drastic shortening of the aliphatic chains and for the decrease of the average molecular weight of the hydrocarbons generated during subsequent maturation. These low molecular weight hydrocarbons are not taken into account in the CH?& eitract (CI6+ hydrocarbons). As a matter of fact, at 350 "C the type Il oxidate has lost 140 mg/g of potential hydrocarbons (HIo- HI(350 "C)) and only generated 64
mg/g of chloroform-extractable compounds. Such comparison clearly shows that even if the two typea of organic matter are characterized by similar initial elemental composition and T,. as well as parallel evolutions during artificial maturation, their detailed behaviors are very different. Consequently,it is necessary to be cautious when interpreting data referring to the evolution of oxidized organic material during thermal maturation. In such cases, the experimental simulation of natural maturation can provide sufficient complementary analytical data in order to study in more details the behavior of oxidized organic matter.
Conclusions The oxidative alteration of type I1 kerogen induces significant modifications in its behavior during thermal maturation: (1)The rates of hydrocarbons and oxygen release are modified. (2) Stable oxygen-bearing functions generated during oxidation induce a delay in the hydrocarbon production and modify the thermostability of the kerogen especially during the catagenetic stage. (3) No regeneration of the petroleum potential lost during oxidation can be expected when pyrolyzing the oxidates. Comparison of a type I1 kerogen oxidate with a type I11 coal of similar elemental composition shows that (1)their respective behaviors during maturation are different; (2) the type of pyrolysis effluents is not similar; and (3) the initial oxygen and hydrogen distribution (type of functionality, aliphatic chain length, etc.) partly controls their evolution during maturation. Acknowledgment. This work was supported by the INSU DBT programs no. 89/3828 and 91/4.08. We thank C. Moreaux and L. GBrard for technical assistance.
Determination of Cloud Point for Waxy Crudes Using a Near-Infrared/Fiber Optic Technique Randy F. Alex,* Bryan J. Fuhr, and Lina L. Klein Alberta Research Council, P.O. Box 8330, Station F, Edmonton, Alberta, T6H 5x2 Canada Received May 13, 1991. Revised Manuscript Received September 9, 1991
A near-infrared (near-IR)/fiber optics method for determining the cloud point of waxy crude oils has been developed. This technique is based on near-IR absorbance changes resulting from wax crystallization during cooling of waxy crude samples. The method shows good repeatability and is supported by viscosity and differential scanning calorimetry measurements. The experimental apparatus allows cloud point measurements at the elevated temperatures (to 135 "C) and pressures (to 14000 kPa) which can be encountered during production and transportation of crude oils.
Crude oils with a high n-paraffin content (waxy crudes) are increasing in commercial importance as conventional petroleum reserves are depleted. However, these waxy crudes can often present special problems during their production and transportation.'2 The tendency for waxy (1) Carnahan, N. F. J . Pet. Technol. 1989,10, 1024. (2) Agrawal, K. M.; Joshi, G. C. Erdol Kohle, Erdgm Petrochem. 1990,
6, 239.
crudes to precipitate wax crystals in pipelines and process equipment can lead to increased pumping costs3 and expensive shutdowns,' respectively. Thus,a measure of wax precipitation propensity for these crudes is an extremely important parameter. Traditionally, the cloud point has been used to measure the onset of wax crystallization in (3) Wardhaugh, L. T.;Boger, D.V. Chem. Eng. Res. Des. 1987,65,74. (4) Majeed, A.; Bringedal, B.; Overa, S. Oil Cas J. 1990,88, 16.
0887-0624/91/2505-0866$02.50/00 1991 American Chemical Society
