Energy & Fuels 1992,6, 560-577
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Effects of Refinery Processes on Biological Markers K. E. Peters,*!+G. L. Scheuerman,i C. Y. Lee,+J. M. Moldowan,§ R. N. Reynolds,i and M. M. Peiias Chevron Overseas Petroleum Inc., Sun Ramon, California 94583-0946; Chevron Research and Technology Company, Richmond, California 94802-0627; and Chevron Oil Field Research Company, Richmond, California 94802-0627 Received February 27, 1992. Revised Manuscript Received June 4 , 1992
Concentrations of biomarkers remaining in straight-run refinery products of crude oil feedstock are mainly controlled by relative volatility. Thermal cracking exerts a second-order control on biomarker compositions of these products. Factors controlling biomarker concentrations and distributions in processed materials are more complex and include volatility, thermal stability, generation from heavier precursors, and the effects of catalysts and hydrogen pressure. Differential volatility of compounds within each biomarker class and sharp temperature gradients defining each distillation cut complicate interpretation of source- and maturation-dependent biomarker parameters used by petroleum exploration geochemists. For example, conventional biomarker parameters could be interpreted to indicate that the residuum is unrelated to and less mature than the feedstock. Partitioning of biomarkers by volatility among distillation cuts affects the compound ratios used for assessment of thermal maturity where the numerator and denominator consist of early- and lateeluting biomarkers, respectively. For example, the residuum, which is enriched in the least volatile biomarkers, shows diasterane/sterane, tricyclic terpanell7a(H)-hopane, and triaromatic steroid TA(I)/TA(I+II) ratios indicating lower maturity than the feedstock. The residuum also shows lower sterane isomerizationratios than the feedstock, possiblydue to release of epimers showing the immature stereochemical configuration (e.g., 20R) from heavier precursors during cracking. Hydrocracked products lack mono- and triaromatic steroids due to their destruction at high temperatures and hydrogen pressures. Increased sterane concentrations in the hydrocracker product could be due to their generation from bound precursors in the feedstock. Source- and maturity-related biomarker parameters for hydrocracker product and TKN feed (gas oil feed for the hydrocracker) are nearly identical. Hydrofining of vacuum gas oil does not significantly alter the concentrations or distributions of terpanes, except Tm [17a(H)-22,29,30-trisnorhopanel.Tm appears less stable to hydrofining than other terpanes. Sterane and aromatic steroid concentrations generally decrease during hydrofining without changes in distribution, except the diamonoaromatic steroids. Fluid catalytic cracking reduces the amounts of most biomarkers without changing their distributions significantly. Fluid catalytic cracking product lacks monoaromatic steroids. Medium coker gas oil is highly volatile and lacks biomarkers, except C19 and CZOtricyclic terpanes. Coking of residuum severely reduces and alters distributions of biomarkers. High concentrations of Tm, 5a,14a,17a(H)-27-norcholestane 20R,and other compounds indicate generation from heavier precursors. Heavy coker gas oil lacks monoaromatic steroids, probably due to low concentrationsin the residuum and subsequent destruction during coking.
Introduction Biological markers (biomarkers) are complex molecular fossils derived from once-living organisms. Biomarkers are ubiquitous in sediments, rocks, and crude oils and they provide information that reduces the risk associated with finding petroleum accumu1ations.l The most common biomarkers used by organic geochemists include terpanes, steranes, and mono- and triaromatic steroids (Chart I). These biomarkers are useful because they are resistant to secondary processes, such as biodegradation, compared to n-paraffins or acyclic isoprenoids. For example, while n-paraffins and acyclic isoprenoids become biodegraded during exposure of seep oils, steranes and other biomarkers commonly survive. Geochemists are frequently asked to distinguish refined from natural products. For example, an oil slick might be + Chevron
Overseas Petroleum, Inc.
* Chevron Research and Technology Co.
Chevron Oil Field Research Co. Moldowan, J. M. The Biomarker Guide; Prentice Hall Publishing Company: Englewood Cliffs, NJ, 1993;363 pp. f
(1) Peters, K. E.;
caused by natural seepage or by a refined product that escaped from a pipeline. Drilling additives, such as diesel or pipe dope, might be mistaken for an oil show, resulting in needless drill stem tests or exploration wells. Biomarkers are potentially useful in distinguishing natural petroleum from synthetic products, but little work has been published on this topic. Because biomarkers are ubiquitous in crude oils, they have potential application as natural tracers to allow better understanding of refiiery processes. The purpose of this work is to better characterize the differences between biomarker distributions in a crude oil feedstock and its refined products. The refined products examined in this study were derived from identical crude oil feedstocks. Mass balance between the feedstock and products allowed better understanding of processes controlling the concentrations and distributions of biomarkers in the samples. The two major categories of refinery streams described in this paper result in straight-run and processed products
0887-0624/92/2506-0560$03.00/00 1992 American Chemical Society
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Effects of Refinery Processes on Biological Markers
Chart I. Some Examples of Biomarkers TERPANES
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(Figure 1). Straight-run streams yield the jet, diesel, gas oil feed for the hydrocracker, vacuum gas oil, and residuum products. The processed streams yield the hydrocracker, hydrofiner, fluid catalytic cracker, and coker products. The hydroclacker reacts gas oil with high-pressure hydrogen over a bed of catalyst pellets to crack the gas oil to lighter products, such as jet and diesel fuels. The hydrofiner reacts gas oil with hydrogen over a bed of catalyst to upgrade the oil by reducing the sulfur and nitrogen content and improving its specific gravity. Fluid catalytic cracking reacts gas oil with fine powdered catalyst a t high temperature to produce gasoline. Unlike the hydrocracker and hydrofiner, no hydrogen is added to the fluid catalytic cracking unit. Delayed coking employs high temperatures but no catalyst to crack heavy residuum into light products, such as gasoline, jet fuel, and gas oils (medium coker gas oil and heavy coker gas oil). Like the fluid catalytic cracker, no hydrogen is added to the coker.
Methods
-
Samples. The samples in this study are divided into three groups (Figure 1): (1)untreated San Joaquin Valley heavy (SJVH)crude oil feedstock;(2)five straight-run products affected STRAIGHT. RUN PRODUCTS
n
50
PROCESSED MATERIALS
--h_7 HYDROCRACKED PRODUCT FEEDS8TOCK HYDROFINED PRODUCT FCC PRODUCT M. COKER GAS OIL H. COKER GAS OIL
Figure 1. Refinery flow diagram showing San Joaquin Valley heavy (SJVH) crude oil feedstock (left), straight-run (center), and processed materials (right). Straight-run products are arranged from most (top) to least volatile (bottom).
Figure 2. True boiling point distillation curvesfor jet fuel, diesel, TKN feed, and VGO from refinery run on SJVH feedstock. primarily by distillation; (3)five processed materials affected by hydrocracking, hydrofining, fluid catalytic cracking, or coking (medium vs heavy coker gas oil) processes. Crude Oil Feedstock. The SJVH crude oil, obtained in Sept 1986 from the Estero pipeline, Morro Bay, CA, consists of 33% South Belridge and 66% Kern River Field oil. This mixture served as the feedstock for all five processed materials in the study. The straight-run products were generated from another batch of feedstock containing the same proportion of South Belridge and Kern River oils as that used for the processed materials. For both the processed and straight-run products, the refinery units from which samples were taken were using only this wellmixed Estero pipeline feedstock. All refinery products were sampled from the Chevron refinery at Richmond, CA. Straight-Run Products. Volatility of the straight-run products decreases in the order jet fuel, diesel, TKN feed (gas oil feed), VGO (vacuum gas oil), and residuum. Jet fuel and diesel are volatile cuts derived by atmospheric distillation of the feedstock. Ninety-five percent of these products were distilled below 346 "C (655O F ; Figure 2). The straight-run TKN feed is an intermediate cut from the atmospheric distillation of the feedstock. Ninety percent of the TKN feed is distilled in the range 306-416 OC (582-781O F ; Figure 2). TKN feed is a Chevron term used to describe this distillation fraction, which consists of gas oil feed for the hydrocracker. For simplicity, we adopt this term for subsequent figures and discussion. Straight-run vacuum gas oil (VGO)is the more volatile fraction of the residuum from the atmosphericdistillation of the feedstock. Maximum temperature of the residuum in the atmospheric distillation column was about 399 "C (750 OF) for about 1 h. When corrected to atmospheric pressure, 90% of the VGO was volatilized from this residuum in the range 384-469 "C (723-877 OF; Figure 2). Processed Materials. Hydrocracked product is obtained from TKN feed using a single-stage hydroprocessing reactor containing hydrocracking catalyst (ICR 126)at a space velocity of 1.5 barrels/h per barrel of catalyst under a hydrogen partial pressure of 2050 psi at 396 OC (745 OF). Hydrofining VGO is obtained from straight-run VGO using a hydrotreating catalyst (ICR 114). The hydroprocessing reactor is run at a space velocity of 2.1 barrels/h per barrel of catalyst
Peters et al.
562 Energy & Fuels, Vol. 6, No. 5, 1992 under a hydrogen pressure of 720 psi at 371 OC (700 O F ) . This product spends less time in contact with the catalyst at a lower temperature and lower hydrogen pressure than the hydrocracked product (above). Fluid catalytic cracking whole product (FCC) results from reaction of VGO on steamed, fresh FCC catalyst at a reactor temperature of 516 OC (960 OF) without hydrogen. Medium coker gas oil is a medium boiling liquid (177-343 "C; 350-650 OF) from coking of the straight-run residuum. No catalyst or hydrogen waa present. Heavy coker gas oil is the heaviest liquid (343 "C+, 650 OF+) from the coking run on the straight-run residuum. No catalyst or hydrogen was present. Most of this liquid was produced in the range 454-493 OC (850-920 OF), similar to most commercial heavy coker gas oils. Residence times in this temperature range are probably less than 5 min. Biomarker Analyses. Gas chromatography (GC) was completed using a 12-m fused silica capillary column with DB-1 stationary phase programmed from 80 to 325 OC at 10 OC/min. Gas chromatography-mass spectrometry (GCMS)analyses were completed on saturated and aromatic hydrocarbon fractions separated from each sample using high-performanceliquid chromatography (HPLC). GCMS was completed using a 60-m fused silica capillary column with DB-1 stationary phase held at 150 O C for 10 min followed by heating to 325 O C at 2 OCImin. Both multiple ion detection (MID,Finnigan MAT 4500 quadrupole system) and metastable reaction monitoring (linked-scan for steranes only, VG Micromass 7070H magnetic sector instrument) analyses were completed in the electron impact mode at 70 eV. Saturated hydrocarbons were examined primarily using mlz 217 (steranes)and mlz 191(terpanes)fragmentograms,while mlz 253 (monoaromatic steroids) and m/z 231 (triaromatic steroids) fragmentograms were used for the aromatic fractions. Detailed procedures and compound identifications are described elsewhere.' Biomarkersin saturated hydrocarbonfractions were quantified using 5B(H)-cholane as an internal standard because it is not present in significant amounts in crude oils, its fragmentation in the mass spectrometer is similar to other steranes, and it does not coelute with other steranes.'s2 We used a series of synthetic aromatic steroids for quantitation of the mono- and triaromatic steroids.' All compounds in Table I are listed in their order of elution from the GC column (scannumbers from GCMS analysis). Error Analysis. Precision values for biomarker concentrations and ratios based on 10 consecutive analyses of the same oil standard' can be used as a general guide to the significance of these results. However, these values must be used with caution because standard deviations about the mean for any parameter can differ between instruments and samples. Examples of standard deviations for several biomarker concentrations and ratios are as follows: ppm C,,, C,,, and C, steranes (9.9,8.0, and 10.2%) C, sterane 20S/(20S+20R) and 14&17@(H)/(&?+aa) (2.0 and 5.6%) ppm C,,, C,,, and C,, monoaromatic steroids (3.3,2.0 and 2.9%) ppm total triaromatic steroids (6.3%)
Concentrationsand Detection Limits. Dataon the amount of sample and the percentage of saturated or aromatic hydrocarbons were used to calculate the absolute concentration of each biomarker in the whole sample (e.g., ppm in jet fuel or ppm in diesel, Table I). Another calculation based on mass balance of the total refinery products waa used to determine the concen(2) Seifert, W.K.;Moldowan, J. M. Geochim. Cosmochim. Acta 1979, 43,111-126.
tration of each biomarker relatiue to that originally present in the feedstock (ppm in crude, Table I). This amount is also expressed as a percentage of the original concentration of each biomarker in the feedstock (percent crude, Table I). Our detection limits for common biomarkers are estimated at better than about 1-2 ppm in the whole sample. The figures and text deal with the concentrations of compounds relatiue to the original feedstock, unless otherwise stated. For example,jet fuel contains an absolute concentration of 1.45 ppm of C19 tricyclic terpane, within detection limits. However, only 0.079 ppm or 0.993 % for the tricyclic terpane remains in the jet fuelrelatiue to the feedstock (ppm in crude and percent crude, respectively, Table I).
Results
Gas Chromatography. The gas chromatogram (GC) of the saturated hydrocarbon fraction of the feedstock (Figure 3) shows (1)a broad range of peaks with retention times ranging from less than that of nC10 to over that of nCso and (2) low, broad humps of unresolved compounds below and above nC22 with no clear maxima. This chromatogram is typical of mildly biodegraded oils where nparaffins and light hydrocarbons have been reduced in abundance but not completely removed. Saturated hydrocarbon fractions of the straight-run products, except residuum, show narrow ranges of peaks and large, unresolved humps with clear maxima, consistent with well-defined ranges of volatility within each distillation cut. Maxima and ranges differ for jet fuel ("nC12; nC5-nCl8), diesel ("nC17; &5-nC20), TKN feed ("nC22; nClgnC2~),and VGO ("nC28; nC22-nC32). The residuum shows a broader range of peaks than the other straightrun products, a homologous series of n-paraffins ('wzC17nCm) maximizing at nC23, and a distinct unresolved hump that increases above -nC28. Gas chromatograms of the saturate fractions of FCC and heavy coker gas oil (Figure 3) are similar to their feed materials (VGOand residuum, respectively)but show more light components. For medium coker gas oil, all components are lighter (less than -nC22) than the residuum feed. Unlike the other processed materials, the distribution of peaks in hydrofined product shows little change from VGO. Biomarker Analyses Chart I shows generalized structures for the compounds described in this paper. Detailed discussions of these compounds and their identification are described elsewhere.' Charts I1 and I11 show mass chromatograms for terpanes and steranes (multiple ion detection, MID), in the feedstock and refined products. Terpanes were quantified using MID. However, metastable reaction monitoring is required to accurately quantify the steranes because of interfering homologs in MID mode. Consequently, not all steranes in Table I can be identified in Chart 111. Chart I11 is intended to show general changes in the distribution of steranes caused by refinery processes. SJVH Feedstock. Mild biodegradation in the reservoirs for the SJVHfeedstock appears to have caused partial loss of n-paraffins without affecting pristane, phytane, steranes, terpanes, or aromatic steroids. Concentrations of many analyzed biomarkers are well above detection limits in the untreated feedstock (Table I). The CZS30nor-l7a(H)-hopane (terpane 17, Table I) is the most abundant biomarker in this feedstock (763.7 ppm).
Effects of Refinery Processes on Biological Markers
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Straight-RunDistillation Products. Jet Fuel and Diesel. Absolute concentrations of all analyzed biomarkers are near detection limits (- 1-2 ppm) in jet fuel, and only the C19 and C20 tricyclic terpanes are present in significant quantities in diesel. The C19 and CZOtricyclic terpanes are more volatile than the other measured biomarkers and are the first terpanes to elute during gas chromatography (compounds 1 and 2, Table I). Depletion of most biomarkers in jet fuel and diesel is due to their relative low volatility in the atmospheric distillation column. Relative concentrations of all biomarkers decrease from feedstock to jet fuel and diesel. For example, only 31.5% of the Clg tricyclic terpane in the feedstock occurs in the diesel (Figure 4, Table I). TKN Feed. Like jet fuel and diesel, TKN feed shows
higher relative amounts of the more volatile biomarkers than the feedstock (Figures 4 and 5). Absolute concentrations of biomarkers in the TKN feed are generally not as low as in jet fuel or diesel, but each compound is much less abundant than in the feedstock. For example, although abundant in the feedstock, the c30 l7a(H)-hopane, C3l to CX, 17a(H)-homohopanes, moretanes, and other heavy pentacyclic terpanes, including 18a(H)-oleananeandgammacerane,areabsentinTKNfeed(Figure4). Second-order deviations from the volatility-controlled curve for steranes in TKN feed (i.e., the saw-tooth pattern, Figure 4) appear to be related to differences in stereochemistry among compounds showing similar scan numbers (e.g., steranes vs diasteranes). All analyzed diasteranes in Figure 4 (e.g., compounds 1-4, 6, 10, 13, 16, 17,
Effects of Refinery Processes on Biological Markers
Energy & Fuels, Vol. 6, No. 5, 1992 567
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Peters et al.
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Energy & Fuels, Vol. 6, No. 5, 1992 669 MONOAROMATICS
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Figure 4. Volatility curves for terpanes and steranes in straightrun refinery products. Amounts of each compound are normalized to the concentration in SJVH feedstock. Scan number is directly related to gas chromatographic retention time and generally increases with volatility. Compounds are numbered along each curve in Figures 4 and 5 (Table I). Shapes of the curves are mainly due to partitioning of biomarkersby volatility during distillationof the cuts. Second-order deviations from the curves are due to other factors,including generationfrom heavier precursors (e.g.,Cze aaa2OR sterane) and differences in thermal stability.
(2) Diasteranes are thermally more stable than sterane5.l~~ This hypothesis requires cracking of some steranes and/or diasteranes over short residence times in the range 306-416 "C. Vacuum Gas Oil (VGO) and Residuum. Like the other distillation cuts, biomarker distributions in VGO and residuum are mainly controlled by volatility. Because of the specific range of volatility of biomarkers, their concentrations relative to the feedstock show roughly bellshaped distributions in the VGO cut (Figures 4 and 5). Biomarker distributions in the residuum are generally skewed toward the heavier, less volatile compounds. Second-order deviations from the VGO curves (sawtooth patterns, Figures 4 and 5) appear to be related to stereochemical differences among the compounds. The observed deviations could result from (1)generation of diasteranes from bound steroid precursors and/or (2) differential stability of steranes and diasteranes. Higher relathe amounts of diasteranes (e.g., compounds 3, 4, 6, 10,13,16,17,20; Figure 4) than steranes (e.g., compounds 5, 7, 11, 12, 14, 15, 18, 19, 21) remain in the VGO. This result is similar to that for TKN feed (Figure 4). Several steranes and diasteranes show higher relative concentrations in the VGO than the feedstock. The high relative concentration of the C26 aaa20R sterane [5a,14a,17a(H)-27-norcholestane16 in both VGO (compound 9, Figure 4; 158.5% of that in feedstock, Table I) and residuum (17.15% , Table I) compared to other steranes and even diasteranes, suggests that some of this (5) Seifert, W.K.;Moldowan, J. M. Geochim. Cosmochim. Acta 1978, 42,77-95. (6) Moldowan, J. M.; Lee, C. Y.; Watt, D. S.; Jeganathan, A.; Slougui, N. E.; Gallegos, E. J. Geochim. Cosmochim. Acta 1991,55, 1065-1081.
---+
Figure 5. Volatility curves for mono- and triaromatic steroids in straight-runrefineryproducts. Shapesof the curvesare mainly due to partitioning of biomarkersby volatilityduring distillation of the cuts. Second-orderdeviations from the curves are due to other factors, including generation from heavier precursors and differences in thermal stability (e.g., dia- vs regular monoaromatic steroids).
compound originates by cracking from heavier precursors during vaccum distillation. This compound does not show unusual abundance in TKN feed, indicating that it is generated at the higher temperatures and residence times characteristic of VGO and residuum formation. There is no reason to suspect that the structure of the c 2 6 aaa2OR sterane could result from higher thermal stability compared to the other c26 steranes and diasteranes in this study. Enhanced c26aaa2OR sterane concentration could result from extensive binding of the steroid precursor with the same stereochemistry to heavier components (e.g., resins or asphaltenes) in the oil followed by release of the sterane upon heating. Such binding might occur through oxygen or sulfur linkage at position C-3' or C-276 in the steroid precursor. Some diamonoaromatic steroids in VGO and residuum (e.g., compounds 10,15,19, and 22; Figure 5) show higher proportions of compounds remaining than the regular monoaromatics (e.g., compounds 16, 17, 20, 21, and 23). Of the diamonoaromatic steroids analyzed, compounds 15,19, and 22 coelute with regular monoaromatic steroids. Only compound 10 is free of this interference, which may explain why it shows the greatest proportion of compound remaining compared to the others (Figure 5). All monoaromatic steroids eluting after compound 7 (Table I) show higher concentrations in VGO than the feedstock, suggesting generation from bound precursors. Triaromatic steroids in VGO show an approximately bell-shaped distribution controlled by the relative volatilities of these compounds. Tricyclic terpanes in VGO (e.g., compounds 5-8,10,11; Figure 4) show higher relative abundance than tetracyclic terpanes (e.g., compounds 4 and 9). As discussed above for diasteranes, generation of tricyclic terpanes may account for these observations. Others have noted an (7) Sinninghe Damsth, J. S.;de Leeuw, J. W. Org. Geochem. 1989,16, 1077-1101.
Peters et al.
570 Energy & Fuels, Vol. 6, No. 5, 1992 TERPANES
2
C
30jI
0
1:1
25 I
C
n Q)
3
6 .-c E
40
35
i
TRIAROMATICS
30 25
ABSENT
20
IN PRODUCT
IN PRODUCT
5
10
o b 0
HYDROCRACKER PRODUCT (ppm in crude) bFigure 6. Relative amounts of biomarkers in TKN feed vs hydrocracker product. Amounts of each labeled compound in Figures 6-9 are normalized to the concentration in SJVH feedstock (Table I). Lines of equal concentration (1:l) are shown for reference.
increase in tricyclic terpanes generated from Monterey Formation rocks during hydrous pyrolysis in the range 300-330 OCe3 18a(H)-Oleanane (compound 36, Figure 4) is enriched in the VGO and residuum compared to other terpanes with similar scan numbers. The C29 moretane and C30 17a(H)-hopane (Table I) elute immediately before and after 18a(H)-oleanane, respectively. Unlike oleanane, these compounds contain five- rather than six-membered E rings (Chart I). The six-membered ring may enhance the stability of 18a(H)-oleanane compared to cZ9 moretane and C ~ 17a(H)-hopane. O Alternately, some 18a(H)oleanane may originate by crackingfrom heavier precursors during the refinery process. The 18a(H)-oleananelC30 17a(H)-hopane ratio first increases and then decreases during burial maturation.8 Processed Materials. Hydrocracked Product. Although significant amounts of mono- and triaromatic steroids occur in the TKN feed, they are absent in the hydrocracked product. These aromatic compounds appear to have been destroyed by the high temperatures and hydrogen pressures of hydrocracking. Steranes are generated during hydrocracking because many show concentrations well above those in the TKN feed (Figure 6). However, the distribution of steranes in the hydrocracked product does not change significantly from that in the TKN feed (Figure6, Chart 1111, apparently because, once generated, steranes are unaffected by hydrocracking. Except for the CISand CZO homologs, tricyclic terpanes are low or absent in the TKN feed and the hydrocracked (8)Ekweozor, C.M.; Udo, 0. T. Org. Geochem. 1988, 13, 131-140.
product (terpanes 1and 2, Figure 6). Some generation of these two terpanes appears to have occurred during hydrocracking because the relative amounts of each compound increase from TKN feed to the hydrocracker product. For example, the amounts of C19 tricyclic terpane in the TKN feed and hydrocracker product account for 25.0% and 76.7%, respectively, of that originally present in the crude oil feedstock (Table I). Hydrofining Vacuum Gas Oil. The concentrations of terpanes decrease in hydrofiner VGO compared to VGO. However, a linear correlation shows that the distribution of terpanes in VGO remains essentially unaltered in hydrofiner VGO (Figure 7), except for Tm [17a(H)-22,29,30trisnorhopane, compound 141. Tm appears to be poorly preserved in hydrofiner VGO compared to other structurally similar terpanes (Figure 7, Chart 11),such as Ts [18a(H)-22,29,30-trisnorneohopane, compound 121, The results suggest that Tm is thermally less stable than Ts, consistent with observations in n a t ~ r eand , ~ ~molecular ~ mechanics calculations.10 Ts/(Ts + Tm) increases from feedstock (0.40)and VGO (0.44)to hydrofiner VGO (0.61). Except for sterane 6 [C28 diaergosfme 20s (24S+24R)1, the steranes show a systematic reduction in relative concentration from VGO to hydrofiner VGO (Figure 7). Hydrofiner VGO contains significant amounts of monoand triaromatic steroids. Apparently the catalyst, lower temperature, residence time, and/or hydrogen pressure allow better preservation of these compounds in the hydrofiner VGO than in the hydrocracked product. (9) Seifert, W. K.; Moldowan, J. M. Geochim. Cosmochim. Acta 1978, 42, 77-95.
(10)Kolaczkowska, E.; Slougui, N.-E.; Watt, D. S.; Marcura, R. E.; Moldowan,J. M. Org. Geochem. 1990, 16,1033-1038.
Effects of Refinery Processes on Biological Markers
1
Energy & Fuels, Vol. 6, No. 5, 1992 671
,
::: 2.0
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/
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HYDROFINER VGO (ppm in crude) Figure 7. Rslative amounts of biomarkers (normalized to feedstock) in VGO vs hydrofiner VGO. Lines of equal concentration (1:l) are-shown for reference.
The concentrations of mono- and triaromatic steroids are reduced from VGO to hydrofiner VGO, apparently without major changes in their distributions (Figure 7). However, hydrofiner VGO shows high diamonoromatic (e.g., Compounds 13,16,18,19, and 22; Figure 7) vs regular monoaromatic steroid concentrations (e.g., compounds 14, 16, 17,20, and 21). Similar preference for diamonoaromatic steroids was noted in VGO compared to feedstock (Figure 5). Some dissimilarities between the VGO and hydrofiner VGO curves in Figure 7 appear to be caused by inaccuraciea related to low concentrations (e.g., monoaromatic steroid 12). Fluid Catalytic Cracking Whole Product (FCC). Most terpanes in FCC product decrease compared to VGO but show a similar distribution (Figure 8). Exceptionsinclude a strong relative increase in the CIS and Cm terpanes (terpanes 1and 2) and a decrease in Tm (terpane 14) from VGO to FCC. The terpane mass chromatogram for FCC product (Chart11) shows a strong increase in unidentified, early-eluting compounds (near the Clg and Cm tricyclic terpanes) compared to VGO. The amount of C19 terpane in the FCC exceeds that in the feedstock, showing that some must be generated during catalytic cracking. The FCC product shows a higher Ts/(Ts + Tm) (0.61) than the feedstock (OM),supporting the lower thermal stability of Tm compared to Ts. Alternatively, some Tm might rearrange to Ts under these conditions. Steranes decrease from VGO to FCC without a significant change in distribution (Figure 8; Chart 111). For example, the (2%aara2OR sterane (sterane 9, Figure 8) is unusually enriched in the VGO (saw-tooth peak in Figure 41, but shows no disproportionate change in abundance from VGO to FCC. The C Zaaa ~ sterane 20S/(20S + 20R)
ratio decreases from feedstock (0.31) to VGO (0.22) and FCC (0.23). The results suggest that the precursor for the C% aaa2OR sterane becomes exhausted during distillation of VGO so that no more of the compound is generated at the higher temperatures of fluid catalytic cracking. Although significant in VGO, monoaromatic steroids are absent in FCC product (Figure 8). Triaromaticsteroids decrease from VGO to FCC but are not absent. The earlyeluting triaromatic steroids (e.g, compounds 1and 2, Table I) show high concentrations in FCC vs VGO compared to other triaromatic steroids, suggesting some generation of these compounds (Figure 8). Medium Coker Gas Oil. The medium coker gas oil lacks saturated and aromatic biomarkers, except for the CISand Cm tricyclic terpanes and a series of unidentified, earlyeluting compounds that are probably terpanes (Chart 11). This results from ineffective volatilization of biomarkers at the low temperatures of the medium coker gas oil fraction (177-343 OC; 351-649 OF). Heavy Coker Gas Oil. Compared to the straight-run residuum, heavy coker gas oil is generally enriched in earlyeluting (e.g., CU, Cm, CB, and c29 tricyclic terpanes; compounds 1, 2, 6-8) and depleted in late-eluting terpanes (e.g., homohopanes; compounds 21,22,24,25, 29, 30,32-36, Figure 9; Chart 11). For example,the Clgtricyclic terpane (terpane 1, Table I) is enriched by about 298% compared to the feedstock and is unusually enriched comparedto the residuum (Figure 91, indicating generation of this compound in the coker. The tricyclic terpane/ l7a-hopane ratio is substantially higher in the heavy coker gas oil (0.139) than in the residuum from which it is derived (0.026). Tm (terpane 14) appears to be generated during coking
Peters et al.
572 Energy &Fuels, Vol. 6, No. 5,1992 TERPANES 160
YI
STERANES 500
I40
0
MONOAROMATICS
,1 {
'"I
60
40
10
I20
200
IM
240
0 6
TRIAROMATICS
4 00
b
0
FCC PRODUCT (ppm in crude) + Figure 8. Relative amounts of biomarkers (normalized to feedstock) in VGO vs FCC product. Fluid catalytic cracking reduces the amounts of terpanes, and severely reduces steranes and mono- and triaromatic steroids. Monoaromatic steroids are absent in FCC, probably due to destruction of small amounts originally in the VGO. Lines of equal concentration (1:l) are shown for reference.
of residuum to heavy coker gas oil (Figure 9). The Ts/(Ts Tm) ratio changes little from crude feedstock (0.40) to residuum (0.36) but drops for heavy coker gas oil (0.13). Data suggest that high concentrations of some steranes in heavy coker gas oil are due to generation from precursors in the residuum. These compounds include c 2 7 5a,14~~,17a(H)20S and -20R, C27 a&32OS and -20R, c28 aaa2OS and -20R steranes, and C27 138,17a(H)-20S and -20R diacholestanes (compounds 11,15,14, 12,18,21,1, 2, respectively). Sterane distributions in the heavy coker gas oil only weakly parallel those in the residuum (Figure 9; Chart 11). Monoaromatic steroids are absent, but heavy coker gas oil contains significant amounts of triaromatic steroids. The residuum feed for the heavy coker gas oil contained low concentrations of monoaromatic steroids, but high concentrations of triaromatic steroids. Early- rather than late-eluting triaromatic steroids in heavy coker gas oil show high concentrations compared to residuum (triaromatic steroids 1 and 2, Figure 9). Mass Balance. Mass balance for all analyzed biomarkers was possible because of complete quantitative information on the feedstock (both lots were well-mixed and contained identical proportions of South Belridge and Kern River oil) and each refinery product (Figure 10). Significant amounts of tricyclic terpanes must be generated from bound precursors during refining. The sum of most terpanes in the refined products is less than that in the feedstock, except for several tricyclics (e.g., terpanes 5-411). Unusually high 18a(H)-oleanane(terpane 36) could be due to greater thermal stability compared to compounds with similar scan numbers. Other com-
+
pounds (e.g., terpanes 4 , 9 , 3 1 ) appear anomalous due to error associated with measurementsof low concentrations (Table I). Steranes eluting before the C27 aaa2OS sterane (sterane 11, Figure 10) show total concentrations in the refined products greater than those in the feedstock, indicating generation of light steranes from bound precursors (largely due to hydrocracking of TKN feed). The saw-tooth pattern resulting from high diasteranes (e.g., steranes 14,6,10,13,16,20) compared to steranes (e.g., steranes 5, 7-9,11,12,14,15,18,19) appemtoresultfromgeneration of these compounds from bound steroid precursors, possibly combined with their greater thermal stability compared to steranes. Monoaromatic steroids eluting after the C17 pregnane analog (monoaromatic steroid 2, Figure 10) show total concentrations in the refiied products greater than in the feedstock, indicating thermal generation from bound precursors. The saw-tooth pattern resulting from unusually high diamonoaromatics (e.g., compounds 13, 15, 18,19,22) could be due to preferential generation of these compounds from precursors or their greater stability compared to regular monoaromaticsteroids (e.g., monoaromatic steroids 14,16,17,20,21,23). The distribution of total triaromatic steroids (Figure 10) is largely controlled by their relative volatility within the VGO cut, where the bulk of these compounds are found (Figure 5). Thermal Maturity Parameters. Maturity parameters for SJVH feedstock were compared to those for the refined products (Table 11). Although the feedstock is a mixture of oils with different thermal histories, this comparison is useful because it provides information on
Energy &Fuels, Vol. 6, No.5, 1992 573
Effects of Refinery Processes on Biological Markers
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-HEAVY COKER GAS OIL (ppm in crude) b Figure 9. Relative amounts of biomarkers (normalized to feedstock) in residuum vs heavy coker gas oil. Coking of residuum to heavy coker gas oil severely decreases and alters the distributions of most terpanes, steranes, and mono- and triaromatic steroids. Lines of equal concentration (1:l)are shown for reference.
the changes in maturity parameters caused by refinery processes. The feedstock has reached equilibrium for the Csz 22S/(22S + 22R) hopane ratio (0.57; 0.57-0.62),11 indicating maturity at least within the early oil-generative stage.lJl The CZSsterane 20S/(20S + 20R) (0.44) and 14@,17@/(@@ + aa) (0.42) ratios are below equilibrium values (0.52-0.55 and 0.67-0.71, respectively), indicating the oil has achieved maturity within the early oil-generative stage,lJ1 This conclusion is consistent with the low tricyclic terpane/l7a-hopane (0.105), Ts/(Ts + Tm) (0.40),diasterane/sterane (1.561, MA(I)/MA(I + 11) (0.09), and TAW TA(1 + 11) (0.07) and high CN moretane/hopane ratios (0.11). The TKN and VGO cuts generally show higher values for thermal maturity parameters than the feedstock. The Cm sterane 20S/(20S + 20R) ratios for TKN (0.57) and VGO (0.59) have reached equilibrium values, while the @@/(@@ + aa)ratios for these cuts (0.58 and 0.54, respectively) have not reached equilibrium values. Higher values for both of the CZSsterane maturity parameters in TKN feed and VGO are consistant with their thermal origin from the feedstock. For example, the 20S/(20S + 20R) ratio is believed to describe the progress of the 20R to 20s isomerization in the C m 5a,14a,17a(H)-steranes during heating. According to this explanation, the rise in the 20S/(20S + 20R) ratio occurs because the R configuration at C-20, which is found in steroid precursors in living organisms, is gradually converted during heating to an equilibrium mixture with the more stable S configura(11)Seifert, W.K.;Moldowan, J. M.Methods Geochem. Geophys. 1986,24,261-290.
tion. Another possible explanation for the increase in these ratios is that the epimers show differing relative thermal stabilities. Preferential thermal destruction of the 20R epimer might explain the increase in the 20S/(20S+ 20R) ratio in the TKN feed and VGO compared to the feedstock. We are unable to distinguish among these possibilities because the concentrationsof the sterane epimersdecrease from the feedstock to TKN feed and VGO partly due to cracking and distillativeloss. For example,the Ca aaa20S and 20R sterane epimers in the feedstock measure 46.84 and 58.25 ppm, respectively, but are reduced to 58.0% and 32.3%, respectively, of these amounts in the VGO (Table I). Except for diasterane/sterane and tricyclic terpanell7ahopanes ratios, the maturity parametera for the TKN and VGO are similar. The TKN feed shows much higher diasterane/sterane (18.88va 3.05) and tricyclic terpane/l7ahopane ratios (6.14 vs 0.547) than VGO. The high values for these ratios in TKN feed are due to the high volatility of this cut and preferential generation of tricyclic terpanes vs l7a-hopanes and diasteranes vs steranes, as described below. Diasterane/sterane ratios do not increase systematically with corrected refinery process temperature as one might expect for parameters controlled by thermal stability: feedstock (1.56),TKN feed (18.881,VGO (3.051, residuum (0.87). At Chevron, we use the ratio of [total C27 to Cn, 138,17a(20S + 20R) diasteranesl/total c 2 7 to CZS 5a,14a,17a and 5a,148,178(20S + 20R)l steranes (Table I). The numerator (steranes 1, 2, 6, 10, 13, 16) and denominator (steranes 11,12,14,15,18,19,21-24) of this
574 Energy & Fuels, Vol. 6, No. 5,1992 120
Peters et al.
,
1101
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I
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TERPANES
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150 140 130
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SCAN Figure 10. Elution order (scan) vs total amounts of each compound in all refinery products normalized to feedstock (percent). Table 11. Biomarker Maturity-Related Parameters for San Joaquin Valley Heavy Crude Oil Feedstock and Refined Products
sample feedstock jet fuel diesel TKN VGO residuum hydrocracker hydrofiner FCC M. Coker gas oil H. Coker gas oil
eteranes terpanes aromatics GCMS C 20S/ C&@/ diasteranes/ C 22S/ C,moretane/ Ts/ tri clics/ TA/ MA(I)/ TA(I)/ no. (208+ 20R) @@+ aa) steranes (228+ 22R) hopane (Ts + Tm) 17axopane (MA + TA) (I+ 11) (I + 11) 0.44 0.42 1.56 0.57 A34 0.11 0.40 0.105 0.93 0.09 0.07 A35 A36 A37 A38 A39 A40 A41 A42 A43 A44
0.57 0.59 0.43 0.58 0.58 0.59
0.58 0.54
0.44 0.55 0.55 0.60
0.68 1.63 18.88 3.05 0.87 16.64 3.22 3.06
0.39
0.42
0.66
6.14 0.547 0.026 9.08 0.552 0.660
0.16 0.69 0.96
0.39 0.02
0.54
0.03
0.57
0.09 0.16 0.10 0.10 0.05
0.53 0.44 0.36 0.60 0.61 0.61
0.52
0.15
0.13
0.139
0.79
0.59 0.54 0.46
0.06
0.17 0.80 0.12 0.03 0.11 0.48 0.36
ratio are dominated by early- and late-elutingcompounds, respectively (Figure 4, Table I). The volatility distributions of TKN feed, VGO, and residuum favor early-, intermediate-, and late-eluting steranes, respectively, which exerts the principal control on the variation in the diasterane/sterane ratio. The diasterane/sterane ratio is actually lower for residuum than feedstock, because the ratio incorporates early-elutingdiasteranes and late-elutingsteranes (Figure 4). The residuum favors late-eluting compounds because of its range of distillation. Rearrangement of steranes to diasteranes is unlikely during distillation of these products because no catalysts are present. In sediments, conversion of sterenes to diasterenes (diasteraneprecursors) occurs by acid-catalyzed clay catalysis.12 Diasterane/sterane ratios increase dramatically after the peak oil-generative stage as shown by
hydrous pyrolysis e~periments.~ At these high levels of maturity, rearrangement of steranes to diasteranes may be possible, even without clays, through hydrogen exchange reactions which are enhanced by the presence of water.13 However, if rearrangement occurred at the higher temperatures of refinery processes, we might expect a systematic increase in the diasterane/sterane ratio from feedstock to TKN, VGO,and residuum. Unlike the other parameters, the monoaromatic steroid MA(I)/MA(I + 11)ratio (Chart I) for VGO (0.02) is lower than for the feedstock (0.09). We use monoaromatics 1 and 2 for MA(1) and all C27 to Cm monoaromatic steroids for MA(I1) (Table I). The volatility distribution in Figure 5 shows that monoaromatics 1and 2 are very low in VGO compared to TKN. Although exposed to higher temperatures than the feedstock, the residuum generally shows isomerization
(12)Kirk, D.N.;Shaw, P.M.J. Chem. SOC.,Perkin Trans. I 1975, 1317-1324.
(13)Ruwlriitter,J.; Aizenshtat,Z.;Spiro, B.Geochim.Cosmochim.1984, 46,2501-2509.
Effects of Refinery Processes on Biological Markers ratios indicating comparable or lower maturity than the feedstock, TKN, and VGO cuts. For example, the C29 sterane 20S/(20S 20R) and /3/3/(/3/3 + aa)ratios for the residuum (0.43 and 0.44) are comparable to the feedstock (0.44 and 0.42) but lower than TKN (0.57 and 0.58) and VGO (0.59 and 0.54). Low isomerization ratios in the residuum can be explained by the release of chemically bound biomarkers from heavy precursors in the oil during high-temperature atmospheric distillation. As previously hypothesized, many bound biomarkers released during laboratoryinduced maturation show stereochemistry indicating low thermal maturity, probably because of steric hindrance of isomerization in the bound ~ t a t e . ~ J ~ Differential volatility of epimers does not explain the observed isomerization ratios. Although the numerator and denominator in the 20S/(20S + 20R) ratio involve early- vs later-eluting peaks [sterane 22/(22 + 2411,the difference in retention time is small. Furthermore, the /3/3/(/3/3 aa)ratio shows the same relationships among the refinery products as the 20S/(20S + 20R) ratio, but does not involve early- vs late-eluting compounds in the numerator and denominator [sterane 23/(22 + 23 + 24)l. The aromatization or TA/(TA + MA) ratio (Chart I) is one exception to the above observations. The residuum shows a TA/(TA + MA) ratio similar to the feedstock but greater than TKN and VGO. This can be explained by the volatility curves in Figure 5. Triaromatic steroids 7 and 9 are used for TA, while monoaromatic steroids 19, 20,22,and 23 are used for MA in the ratio (Table I). The residuum shows high and low relative amounts of MA and TA, respectively, compared to TKN feed and VGO. Hydrocracking of TKN results in few changes in the terpane and sterane maturity parameters. For example, TKN and hydrocracked product show similar ratios for C29 20S/(20S + 20R) (0.57 and 0.58, equilibrium), /3@/(/3/3 aa)(0.58 and0.55), diasterane/sterane (18.88and 16-64), tricyclic terpanell7a-hopane (6.14 and 9.081, and Ts/(Ts + Tm) (0.53 and 0.60, respectively). This comparison is not possibleusing aromatic steroid maturity ratios, because these compounds are destroyed by hydrocracking. Hydrofining of VGO results in little change among biomarker maturity parameters, except C32 22S/(22S + 22R) (0.59-0.46) and Ts/(Ts + Tm) (0.44-0.61, respectively). We are unable to explain these differences. Fluid catalytic cracking of VGO results in little change or a slight increase in thermal maturity parameters. For example, FCC appears more mature than VGO based on /3/3/(/3/3 + aa)(0.60 vs 0.54), tricyclic terpane/l7a-hopane (0.660 vs 0.5471, C30 moretane/hopane (0.05 vs 0.09), Ts/ (Ts + Tm) (0.61 vs 0.44) and TA(I)/TA(I + 11) (0.48 vs 0.12, respectively). FCC and VGO show similar diasteranel sterane ratios (3.06 vs 3.05). Note that some ratios, like 20S/(20S + 20R) are similar for FCC and VGO, but only because they have reached equilibrium values. Medium coker gas oil shows unreliable biomarker maturity ratios because of low concentrations. In general, heavy coker gas oil shows maturity parameters comparable to those for the residuum. For example, the anomalously low 20S/(20S + 20R) and @@/(@/3 + aa) ratios in the residuum (0.43 and 0.44, discussed above) are similar to those in the heavy coker gas oil (0.39 and 0.42, respectively). Some parameters indicate the residuum is less mature than heavy coker gas oil, such as tricyclic terpane/l7a-
Energy & Fuels, Vol. 6, No. 5, 1992 676 Regular Slrrmr Melrrtrblr Data
c2s
A\
+
+
+
(14)Peters, K.E.;Moldowan, J. M. Org. Geochem. 1991, 17, 47-61.
/.pa
c 21
c2,
Figure 11. Ternary diagram showing the relative abundances of C27, (228, and C ~ regular S steranes [5a(H),17a(H),20S+ 20R + 20Rl in saturated fractions of and 5a(H),14@(H),17B(H),20S SJVH feedstock and refined products determined by metastable reaction monitoring-gas chromatography-mass spectrometry (M*+ m/z217). Dashed line in Figures 11-13 is for reference only. R = H, CHs, or C2Hs.
-
..."
' I .
C2,L'
'
'
'
'
,
lC2S
,
Figure 12. Ternary diagram showing relative abundances of c27, CB, and C a rearranged steranes [13B(H),17a(H)-diasteranesl in saturated fractions of SJVH feedstock and refined products determined by metastable reactionmonitoring-gaschromatography-mass spectrometry (M'+ m/z217). R = H, CH,, or CzHs.
-
Monorromatk Storold Data
c 28
A / \
0
x TKNfemd
A34 A37
0 Mo
A38
f M 8 W
c27
Figure 13. Ternary diagram showing relative abundances of (227,Cm, and C a monoaromatic steroids in aromatic fractions of SJVH feedstock and refined products determined by gas chromatography-mass spectrometry in selected ion mode (m/z253). R = H, CH3,or CZH~. hopane (0.026 vs 0.1391, and TA(I)/(I + 11) (0.03 vs 0.36), while others indicate the opposite, such as diasterane/ sterane (0.87 vs 0.66), and Ts/(Ts + Tm) (0.36 vs 0.13, respectively). CorrelationParameters. Dramatic changes in biomarker distributions during refinery processesappear largely controlled by differential volatility, as indicated in ternary diagrams for C27 to C29 steranes, diasteranes, and monoaromatic steroids (Figures 11-13, Table 111). These diagrams are normally used to show genetic relationships among natural oils and bitumens.' In general, the plot locations
576 Energy & Fuels, Vol. 6, No. 5, 1992
Peters et al.
Table 111. Biomarker Source-Related Parameters for San Joaquin Valley Heavy Crude Feedstock and Refined Products steranes
diasteranes terpanes aromatics GCMS GO/ Czd CZSI Czd CZSJ gamm. hopane MAGl/ M A C d TAC2sS/ TAC2,R/ no. (CzrC30) (cz7-C~) ( C z r C d (Cz.29) (CzrCzs) index index (CzrCz~)( C Z ~ Z S )(C& C& sample 7.0 A34 0.049 0.46 0.24 0.26 0.22 1.674 0.21 0.28 0.94 1.80 feedstock jet fuel A35 A36 diesel A37 0.79 0.06 0.46 0.08 0.40 0.13 3.23 5.79 TKN A38 0.028 0.55 0.17 0.24 VGO 0.24 1.109 2.0 0.22 0.27 1.55 2.98 A39 0.100 0.32 0.37 0.17 0.32 3.037 9.3 0.16 0.36 0.62 2.02 residuum A40 0.78 0.07 0.45 0.09 hydrocracker A41 0.030 0.54 0.18 0.24 0.23 1.171 1.6 0.21 0.29 1.77 2.87 hydrofiner A42 0.029 0.56 0.18 0.23 0.25 1.061 1.8 3.38 3.25 FCC M. Coker gas oil A43 0.033 0.59 0.18 0.24 H. Coker gas oil A44 0.27 1.649 5.1 0.14 0.54 1.01 2.00
for oils and bitumens do not change appreciably throughout the oil-generative stage. Steranes. No clear correlation exists between sterane compositions of the feedstock and any refinery product (Figure 11). Jet fuel and diesel contain no steranes. The TKN and hydrocracker products plot in nearly identical positions in Figure 11and are strongly enriched in c27 and depleted in c 2 9 steranes compared to the feedstock. They also show similar tricyclic terpane distributions (Chart 11). The TKN cut favors early-eluting compounds. Proportionally more C27 steranes escape to the TKN cut during distillation than VGO or residuum (e.g., Figure 4). Hydrocracking of TKN feed does not significantly alter sterane distributions. The VGO,hydrofiner, and FCC products show similar sterane distributions enriched in C27 and depleted in c29 steranes compared to the feedstock (Figure 11). Hydrofining and fluid catalytic cracking do not significantly alter the sterane distributions in the VGO. The enrichment of C27 and depletion in C29 steranes in the VGO is opposite to that observed in the residuum. VGO and residuum are from the top and bottom of the vacuum distillation column, explaining these differing compositions. Heavy coker gas oil is enriched in c27 and depleted in C2s steranes compared to both the residuum and feedstock (Figure 11). Figure 9 indicates this is due to alteration of these distributions during coking. Also, low steranes in the heavy coker gas oil may explain some of the scatter in Figure 11. Others suggest that the distributions of c27 to c29 steranes in both natural and synthetic fuels can be used to determine their source materials.l5 Their study materials included whole crude oils and the methylene chloride-solubleportion of the entire liquefaction or retort product. Our study deals with refinery products that were separated by distillation prior to any further treatment. The differing results of these studies support the critical role of volatility in determining biomarker compositions of the products. Diasteranes. Like the sterane results, TKN feed and hydrocracked product show nearly identical diasterane distributions characterized by higher c27 and lower c29 diasteranes compared to the feedstock (Figure 12). Hydrocracking does not significantly alter the diasterane composition of TKN feed (Figure 6). Diasterane distributions for VGO, hydrofiner VGO, FCC, and heavy coker gas oil are similar to those for the feedstock. The residuum shows less c27 and more c29 diasteranes than the feedstock, like the results for steranes. (15) Strachan, M. G.; Alexander, R.;Kagi, R.I. Fuel 1989,68,641-647.
Monoaromatic Steroids. The monoaromatic steroid distributions (Figure 13) indicate a close relationship between the feedstock, VGO,and hydrofining product. Like the results for steranes and diasteranes, monoaromatic steroids in the TKN feed are higher in the C27 and lower in the C29 homologs than the feedstock. However, hydrocracking of TKN, which does not appear to affect sterane or diasterane distributions (Figures 11 and 12), results in a monoaromatic steroid distribution that is similar to the feedstock (Figure 13). The similarity between feedstock and hydrofiner product monoaromatic steroid distributions appears fortuitous, because the hydrofiner product was altered from the composition of TKN feed. Like the sterane and diasterane results, the residuum is enriched in the C29 homologs of the monoaromatic steroids compared to the feedstock. Other Parameters. Gammacerane and hopane indexes and triaromatic steroid TA C~SS/C~SS and TA C27R/C& ratios show values for feedstock and refined products generally consistent with the importance of volatility in controlling these ratios (Table 111). For example, TKN feed shows higher TAC&3/C& andCnR/C&ratios (3.23 and 5.79) than feedstock (0.94 and 1.80) consistent with a preference for early-eluting compounds.
Conclusions Apart from feedstock composition, biomarkers in straight-run refinery products are mainly controlled by relative volatility. Thermal cracking exerts a secondary influence. Differential volatility of compounds within each biomarker class and sharp temperature gradients defining each distillation cut can complicate interpretation of source- and maturation-dependent biomarker parameters. Jet fuel and diesel distillation cuts lack commonly analyzed biomarkers, except for highly volatile CIS and C20 tricyclic terpanes. The distillation range of TKN feed (gas oil feed for hydrocracker) overlaps only the most volatile biomarkers. Thus, TKN feed is low in biomarkers and shows a preference for light terpanes, steranes, and mono- and triaromatic steroids. TKN feed is enriched in c27 vs c29 steranes, diasteranes, and monoaromatic steroids compared to the feedstock, obscuring their genetic relationship on C27-C28-C29 ternary diagrams. Most biomarker maturity parameters for TKN feed show higher values than the feedstock, partly because the numerator and denominator of these ratios commonly consist of early- vs lateeluting compounds. The volatility range of vacuum gas oil approximately corresponds to that of most biomarkers with minimal
Effects of Refinery Processes on Biological Markers
preference for light or heavy homologs. VGO contains more biomarkers than TKN feed, but less than the feedstock. VGO is only slightly enriched in c27 vs C Z ~ steranes compared to the feedstock on the CzrCza-Czs ternary diagram. VGO and feedstock show nearly identical CzrCza-Czg diasterane and monoaromatic steroid distributions. Others have observed that biomarkers can be used to determine the source for synthetic liquid fuels generated from coals or oil shales.15 Although the feedstocks and processes used to generate synthetic liquids and VGO differ, we believe that both products contain source-specific biomarker distributions that have been little altered by distillation. Most biomarker parameters indicate that VGO is more mature than feedstock, except the monoaromatic steroid MA(I)/MA(I + 11)ratio. This difference can be explained by the volatility curvefor VGO, which shows depletion of early-eluting MA(1) compared to the feedstock. The volatility range of the residuum overlaps only the least volatile biomarkers. Residuum is rich in biomarkers but contains less than the crude feedstock. Residuum is enriched in C29 vs C Zsteranes, ~ diasteranes, and monoaromatic steroids (opposite TKN feed) compared to feedstock on CZ&Z~-CZSternary diagrams. On the basis of conventional interpretation of the ternary diagrams, residuum and feedstock appear unrelated. Residuum shows many biomarker parameters indicating similar or lower maturity than the feedstock. Two explanations are likely: (1)Many maturity ratios place the more and less volatile components in the numerator and denominator, respectively. Because the residuum preferentially retains heavier homologs, these maturity ratios might be expected to drop compared to the feedstock. (2) Ratios based on proposed isomerization reactions may remain low because of release of sterically protected biomarkers during heating of the residuum. These new products may show the immature stereochemistry as suggested by other ~ t u d i e s . ~ J ~ Hydrocracked product lacks mono- and triaromatic steroids due to their destruction a t high temperature and hydrogen pressure. Hydrocracked product contains more steranes than the feedstock, indicating generation of these compounds from heavier precursors. Little change in the distribution of terpanes or steranes occurs during hydrocracking. Source- and maturity-related biomarker parameters for the hydrocracked product and TKN feed are nearly identical.
Energy & Fuels, Vol. 6, No. 5, 1992 577
Although biomarker concentrations decrease, hydrofining of VGO does not significantly alter the distribution of most biomarkers, except Tm and diamonoaromatic steroids. Moat thermal maturity parameters for hydrofining product and VGO are similar except Ts/(Ts + Tm). Tm appears less stable to hydrofiningthan other terpanes. The CzrCza-Czs sterane and diasterane distributions for hydrofining product and VGO are nearly identical. However, the CzrCzs-Cz~monoaromatic steroid composition for hydrofining product differs from VGO and is similar to the crude feedstock. Fluid catalytic cracking (FCC) product lacks monoaromatic steroids and contains lower amounts of most terpanes, steranes, and triaromatic steroids compared to the VGO. Concentrations of biomarkers are generally slightly lower than hydrofiningVGO, which is also derived from VGO. Parameters generally indicate the FCC is slightly more mature than VGO, except for similar diasterane/sterane ratios. FCC, VGO, and hydrofiner VGO contain nearly identical distributions of CzrCmC29 steranes and diasteranes. Medium coker gas oil lacks commonly analyzed biomarkers, except for highly volatile C19 and CZOtricyclic terpanes. Heavy coker gas oil lacks monoaromatic steroids. Inaccuracies related to measurement of low steranes may explain the apparent enrichment in c27 vs C29 steranes for heavy coker gas oil compared to the residuum and feedstock. The CZTCZ&~ diasterane distribution for heavy coker gas oil is similar to that for residuum and feedstock. There are few consistent changes in thermal maturity parameters between the residuum and heavy coker gas oil. Unlike all other refinery products, the distributions of biomarkers in the heavy coker gas oil do not parallel those in the residuum. For example, residuum favors heavy homologs of terpanes and triaromatic steroids, while the heavy coker gas oil favors light homologs. This explains the higher tricyclic terpane/ l7cu-hopane and triaromatic steroid TA(I)/TA(I + 11) ratios for heavy coker gas oil compared to residuum.
Acknowledgment. We thank M. M. Boduszynski, R. M. K. Carlson, and three Energy and Fuels reviewers for their comments on this work.
