2 Chemistry and Transport of Petroleum Hydrocarbons 3800 B.C. First documented use of asphalt for caulking reed boats 2.1 INTRODUCTION An understanding of the chemistry and transport of petroleum hydrocarbons provides the foundation for forensically reviewing information dealing with petroleum hydro- carbon contamination. This chapter provides basic terminology and concepts asso- ciated with the transport and fate of crude oil and refined products in the subsurface. 2.2 CHEMISTRY OF CRUDE OIL There are over one million types of hydrocarbons in crude oil, ranging from light gases to heavy residues. No two crude oils are identical. Crude oil is defined by Philip (1998) as… …extremely complex mixtures of saturated and aromatic hydrocarbons, ranging from C 1 to C 100 or higher, plus a wide variety of compounds containing nitrogen, sulfur, and oxygen. In addition, there is also a fraction called the asphaltene fraction which is basically insoluble in n-pentane and contains a very complex matrix of high molecular weight polar compounds. In most cases, 90 to 98% by weight of crude petroleum consists of hydrocarbons, while the remaining materials include sulfur, oxygen, nitrogen, and other organic compounds. Variations in crude oil composition occur due to the nature of the source of the organic material, the geologic and thermal history, chemical changes that occur during oil formation and migration, and chemical alteration due to biodegradation, oxidation, or selective dissolution. Despite wide variations in the chemistry of crude oil, the elemental compositions fall within a narrow range of elements, as shown on Table 2.1 (Neumann et al., 1981). Crude oils have normal paraffins (n-paraffins) ©2000 CRC Press LLC ranging from C 1 to C 40 . Although higher carbon numbers exist in crude oils, most crude oils fall within the C 5 to C 30 range (Schmidt, 1998). The predominant hydrocarbon classes that comprise crude oil are straight or branched- chain alkanes, cycloalkanes, and aromatics. Alkanes (paraffins) are saturated hydrocar- bons. Linear or normal alkanes (n-alkanes) ranging from C 1 to C 40 have been identified in crude oil and usually comprise 15 to 20%. In general, the most abundant alkanes in crude oil are the low-molecular-weight normal alkanes (C 5–10 ). Normal alkanes (n-alkanes) are linear chains of carbons linked by single covalent bonds. Isoalkanes are hydrocarbons containing branched carbon chains. The highest concentration of isoalkanes in crude oils is in the C 6 to C 8 range. Crude oil can contain 10 to 15% isoalkanes. Cycloalkanes are similar to alkanes except that cycloalkanes consist of rings of carbon atoms joined by single atomic bonds. Cycloalkanes are abundant in crude oils and can comprise up to 30 to 40% by weight. The most abundant cycloalkanes (also called naphthenes) are the single-ring cyclopentanes (C 5 H 10 ) and cyclohexanes (C 6 H 12 ). Steranes and triterpanes are complex cycloalkanes often used as markers to identify the source and age of crude oil (Hughes and Holba, 1988; Seifert and Moldowan, 1978; Stout et al., 1999). Aromatic hydrocarbons consist of rings of six carbon atoms that are unsaturated (i.e., they do not contain the maximum number of bonded hydrogen atoms). Aromat- ics include the BTEX (benzene, toluene, ethylbenzene, and total xylenes) and poly- nuclear aromatic compounds (PNAs). Aromatic hydrocarbons contain carbon atoms linked with double bonds, the simplest being benzene (C 6 H 6 ). Each hydrogen atom on the aromatic ring can be replaced with an alkyl group (CH 3 ) which results in compounds such as toluene with one alkyl group attached to the benzene ring. Benzene rings can be linked to other benzene rings to form compounds such as biphenyls or terphenyls. When two or more benzene rings are fused, polynuclear aromatic hydrocarbons (also known as polycyclic aromatic hydrocarbons, or PAHs) are formed (see Section 4.10 in Chapter 4). Polycyclic aromatic hydrocarbons are compounds that originate from crude oil and many pyrolysis processes. Polycyclic aromatic hydrocarbons are of concern because of their genotoxic properties. Naph- thalene (C 10 H 8 ) is a lower molecular weight example and is generally considered to be a polycyclic aromatic hydrocarbon, although it has only two aromatic rings. Other non-hydrocarbon components in crude oil include sulfur, oxygen, and nitrogen. TABLE 2.1 Elemental Composition of Crude Oil Element Composition (%) Carbon 84–87 Hydrogen 11–14 Sulfur 0–3 Nitrogen 0–1 Oxygen 0–2 ©2000 CRC Press LLC Sulfur is typically the most abundant element and may be present in several forms, including elemental sulfur, hydrogen sulfide, mercaptanes, and thiophenes (i.e., hydrogen molecules with bonded sulfur atoms). The sulfur content in most crude oils varies from about 0.1–3% for some of the heavier oils to 5–6% for bitumen. Sulfur does not decompose during the distillation process. The majority of sulfur is, there- fore, present predominately in the higher molecular weight fractions and becomes concentrated in the higher weight refined products. The analysis of the sulfur content of crude and refined products, such as diesel, can be used to provide evidence to distinguish between multiple sources. The sulfur content of a petroleum hydrocarbon is determined using standards such as American Society for Testing Materials (ASTM) D-124, D-1552, and D-4294. Oxygen reacts with hydrocarbons to form compounds such as phenols, cresols, and xylenols. Nitrogen can bond with hydrocarbon molecules in crude oil to form small concentrations of pyrrole, pyridine, and quinoline. Metals are present in crude oils, although usually in small amounts. Metals can occur as inorganic salts, metallic soaps, and organometallic compounds. In some instances, sodium arsenite and arsenic trioxide are added to oil pumping wells to inhibit corrosion (Rohrbach et al., 1953; Wellman et al., 1999). The presence of arsenic in crude oil may, therefore, provide a means for identifying the origin of the crude oil. 2.3 CHEMISTRY OF REFINED PRODUCTS The chemistry of a refined petroleum product is the result of the composition of the crude oil and the refining process. The term “refined products” refers to those petroleum hydrocarbons that are blended and to which additive packages are in- cluded. Examples of refined products include gasoline, aviation fuels, jet fuel, and the newer formulations of diesel fuels (Harvey, 1998). Major refinery processes that affect product chemistry are (Speight, 1991): • Separation of the crude oil into various fractions • Conversion of marketable portions of the crude oil • Finishing of the various product streams Separation and removal of the various portions of crude oil have historically been accomplished via distillation. The three products created via distillation are naphtha, middle distillates, and residual hydrocarbons. Naphtha, with a boiling range of 90 to 190∞C, includes gasoline, which is further processed for octane improvement. The middle distillate fractions are separated into kerosene (light-end) and diesel range (heavy-end) products The light-end middle distillates (boiling ranges from 150 to 260∞C) include kerosene, mineral spirits, Stoddard solvent, jet fuels, and diesel No. 1. Stoddard solvent was used extensively in the first half of this century for degreasing but was replaced by chlorinated solvents such as trichloroethylene due to the poten- tial fire hazards associated with Stoddard solvent (Stewart et al., 1991). Examples of heavy-end products are Bunker fuels, heavy fuel oils, and asphalt. Examples of chro- matograms for mineral oil, Stoddard solvent, and kerosene are shown in Figure 2.1. ©2000 CRC Press LLC FIGURE 2.1 Chromatograms of mineral oil, Stoddard solvent, and kerosene. (From Bruya, J., Chromatograms, Friedman and Bruya, Seattle, WA, 1999. With permission.) ©2000 CRC Press LLC Heavy-end middle distillates with boiling ranges of 190 to 400∞C are processed to produce diesel fuel No. 2 and heating oils (Kaplan et al., 1995). Table 2.2 summarizes key distilled products, their distillation temperature range, and carbon range (Galperin, 1997; Schmidt, 1998). While the distillation temperature and Ameri- can Petroleum Institute (API) gravity of hydrocarbons provide useful information in the refining process, they can provide corroborative evidence in distinguishing among multiple sources of fuel releases. API gravity is defined in Equation 2.1 as: API gravity = 141.5/P – 131.3 (Eq. 2.1) where P is the specific gravity of the crude oil or refined product at 60∞F. Evidence used to distinguish among sources of diesel, gasoline + diesel + jet fuel, and gasoline at a refinery is shown in Figure 2.2 as a function of API gravity. The API gravity of each of the various fuels stored at the refinery were known, thereby providing a baseline for comparison. The use of the distillation temperature of a fuel to distinguish among multiple sources (degraded gasoline and a gasoline + diesel + jet fuel mixture) is shown on Figure 2.3. For the free product samples collected from the groundwater table shown in Figure 2.2, the initial boiling point (IBP) and final boiling point (FBP) of the fuels were known, thereby allowing correlation of the IBP and FBP of the samples to specific locations on the refinery. The evolution of crude oil refining over time has resulted in different products and blends of refined product. The unit process and the waste streams from these processing changes are helpful in age-dating a product and/or bracketing a time frame when certain refinery processes and their associated waste products were produced. Table 2.3 summarizes some of the key historical changes in petroleum refining (Gibbs, 1990; Harvey, 1998). 2.3.1 GASOLINE Gasoline is composed of low-boiling hydrocarbons in the C 5 to C 10 –C 12 range that are ignitable in an internal combustion engine. On a chromatograph, fresh gasoline TABLE 2.2 Distillation Temperature and Carbon Range of Distilled Products Distillation Temperature Product (C∞) Carbon Range Gasoline 30–200 C 5 –C 10/12 Naphtha 100–200 C 8 –C 12 Kerosene and jet fuels 150–250 C 11 –C 13 Diesel and fuel oils 160–400 C 13 –C 17 Heavy fuel oils 315–540 C 19 –C 25 Lubricating oils 425–540 C 20 –C 45 ©2000 CRC Press LLC exhibits an asymmetric distribution pattern from the CH 1 (methylcyclohexane) to CH 7 (a heptylcyclohexane) range, with the CH 2 peak being the most abundant and the peaks CH 2 to CH 7 decreasing rapidly in intensity (Galperin, 1997). Gasoline has a boiling-point distribution from about 120 to 400∞F. As a result of the preferential partitioning of low-boiling-temperature compounds found in gasoline, the concentra- tion of the BTEX components can be as high as 1 to 4% for benzene and 3 to 20% for toluene. Gasoline blending has changed, in part, to create fuels with different octane ratings. Examples of gasoline grades are summarized in Table 2.4 (Harvey, 1998). Gasoline blends often reflect the level of refining. A premium-grade gasoline, for example, is a FIGURE 2.3 Use of initial (IBP) and final boiling point (FBP) temperatures to identify fuel types. FIGURE 2.2 Use of API gravity to distinguish between fuels. ©2000 CRC Press LLC more tightly regulated blend than a mid-grade or regular gasoline. Chromatograms of gasoline grades and blends are shown in Figure 2.4 (Zemo et al., 1993). Changes in the octane ratings of different gasoline grades include a 65 to 75 octane rating in 1910, an average octane rating of 82 in 1946, and an average octane rating of 96 in 1968 (Gibbs, 1990). The significance of these different gasoline grades and octane ratings over time is that it is unlikely that forensic testing can identify a gasoline grade once it has entered the subsurface. Compounds used to provide higher octane ratings, however, can be identified on a chromatogram. Examples include iso- octane, toluene, ethylbenzene, xylenes, and trimethylbenzene. For example, a pre- mium-grade, 1994 gasoline tends to have a high percentage of iso-octane and aromatics. The greater the combined percentage of iso-octane and aromatic com- pounds, such as toluene, the higher the octane and fuel quality and, therefore, the more likely it is that the product was refined and blended. TABLE 2.3 Chronology of Key Changes in Petroleum Refining in the U.S. Date Key Refinery Process 1910 Straight run (distilled) products produced; 65–75 octane rating 1913 Dubbs thermal cracking process introduced 1920 Coking introduced 1923 Lead introduced in gasoline to minimize backfiring 1926 Lead anti-knock additive introduced 1928 Lead scavengers ethylene dibromide and ethylene dichloride introduced 1929 Regular and premium gasoline sold 1936 Fluid catalytic cracking introduced 1938 Alkylation introduced 1940 Reforming introduced 1959 Hydrocracking introduced 1970–74 More olefins added to gasoline 1980 Lead regulations 1990 Advent of environmental regulations of sulfur, aromatics, and oxygenates TABLE 2.4 Grades of Gasoline Leaded Gasoline Unleaded Gasoline Super premium leaded Premium or supreme unleaded Premium or supreme leaded Mid-grade unleaded “Super regular” leaded Regular unleaded Regular leaded Economy leaded Regular low lead ©2000 CRC Press LLC Refined gasoline contains olefins (alkenes and alkynes), while crude oils and virgin naphthas do not. As a result, olefins are useful for distinguishing between refined and crude oils. Olefins are products of the catalytic cracking process. Olefins are identified on chromatograms as a cluster of small peaks to the right of the C 6 peak (Schmidt, 1998). Alkynes (acetylenes) are also not normally found in crude oil. Another indicator used to distinguish between refined and unrefined products is the FIGURE 2.4 Gasoline chromatograms. (From Zemo, D. and T. Graf, in Proc. of the 1993 Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Restoration, November 10–12, Houston, TX, Ground Water Management Book 17, National Ground Water Association, Dublin, OH, 1993, pp. 39–54. With permission.) ©2000 CRC Press LLC presence of methylcyclohexane (MCH). Methylcyclohexane is abundant in unre- fined gasoline range hydrocarbons and naphthas. In general, the greater the concen- tration of methylcyclohexane, the less the product has been refined, as well as the lower the octane rating of the gasoline. 2.3.2 DIESEL Diesel consists of hydrocarbons in the C 11 to C 18–27 range. Depending on the grade of diesel, it contains a high concentration of cycloalkanes and smaller amounts of aromatic compounds (i.e., BTEX). Diesel tends to have greater concentrations of polycyclic aromatic hydrocarbons than does gasoline. The changing formulation of diesel provides opportunities for dating a release. Prior to 1975, diesel was primarily straight chained, while post-1975 diesel was thermally cracked. This distinction can be determined analytically with only several milliliters of product, thereby providing a bracket of time when the product was available. Chromatograms of a diesel fuel No. 1 and No. 2 are shown in Figure 2.5. FIGURE 2.5 Chromatograms for diesel fuels No. 1 and No. 2. (From Bruya, J., Chromato- grams, Friedman and Bruya, Seattle, WA, 1999. With permission.) ©2000 CRC Press LLC Diesel is available in various grades. Diesel, kerosene, and the lighter distillates contain various amounts of the BTEX aromatics up to 1500 parts per million (ppm) (Dunlap and Beckmann, 1988). The composition and characteristics of diesels and middle distillate products are described in Table 2.5 (Havlicek, 1986; Kaplan et al., 1995; Kaplan and Galperin, 1996). A petroleum forensic laboratory to needed to distinguish between fuels that are similar. Figure 2.6 illustrates chromatograms for jet fuel No. 4 (JP-4) and jet fuel No. 5 (JP-5) to show the chromatographic similarity of these two fuels (Bruya, 1999). TABLE 2.5 Products, Synonyms, and Characteristics of Diesel and Jet Fuels Product and Synonym(s) Composition and Characteristics Diesel No. 1 Similar in composition to a blend of kerosene and diesel No. 2. Diesel No. 1 is manufactured in cold climates and is also sold in warm climates when a refinery desires to blend its kerosene with more expensive diesel No. 2. Diesel No. 1 exhibits an alkylcyclohexane pattern on a mass chromatograph in the range from CH 1 to CH 14 , with a maximum at CH 5 . Diesel No. 2 Automotive diesel. Straight-run or catalytically cracked petroleum distillate with a typical carbon range of C 8–9 to C 24–27 and a boiling range of approximately 163 to 382∞C. Includes straight-run kerosene, middle distillate, hydro-desulfurized middle distillate, and light catalytically and thermally cracked distillates. Formulated for use in atomizing-type burners. BTEX components can be present in small amounts. Using gas chromatography/mass spectrometry (GC/MS), diesel No. 2 shows a range of alkylcyclohexanes from CH 1 to CH 14 , maximizing around the CH 9 and CH 10 peaks. Characterized by a smooth n-alkane distribution pattern. Diesel No. 4 Railroad diesel. A straight-run or cracked petroleum distillate with a typical carbon range of C 11 to C 30 . Used without preheating in commercial or industrial burners that can accommodate a higher viscosity diesel such as diesel No. 4. Diesel No. 5 A fuel comprised primarily of straight-chained hydrocarbons. Diesel No. 5 is a residual fuel that often requires preheating for handling. Bunker C (heating A residual fuel used in commercial and industrial heating. Bunker C requires preheating for storage and for burning. Sulfur is often found in higher concentra- tions than in other diesels, unless they are deliberately extracted. Bunker C is the primary fuel for steam-powered ships and for onshore power-generation plants and is primarily a mixture of diesel No. 1 and No. 2 and residual oil. Bunker C is a distillation residue of crude oil and contains biomarkers such as terpanes and steranes. Bunker C has a hydrocarbon range from C 9 to about C 36 and a boiling point range of about 340 to 1050∞F. Kerosene A straight-run distillate with hydrocarbons in the C 9/10 to C 16 range. A light-end middle distillate used in vaporizing-type burners where the fuel is ignited by contact with a heated surface or radiation. Consists primarily of paraffins with smaller amounts of naphthalene and aromatic hydrocarbons. The carbon distribution peaks around C 12 to C 13 . It is similar in composition to JP-5 and JP-6 jet fuels. oil or diesel No. 6) (No. 1 fuel oil) ©2000 CRC Press LLC [...]... (C18H 12) Fluoranthene (C10H10) Fluorene (C13H10) Naphthalene (C10H8) Phenanthrene (C14H10) Pyrene (C16H10) a Soil Groundwater 120 –384 96– 528 72 24 0 168–6 72 299 24 48 120 0–11040 1368– 12, 720 8904 24 ,000 3360–10,560 768–1440 398–11 52 384–4800 5040–45,600 24 0–17 ,28 0 168–6 72 144–54 72 336–8640 590–4896 24 00 22 ,080 27 36 25 ,440 17,808–48,000 6 720 21 , 120 1536 28 80 24 –61 92 768–9600 10,080–91 ,20 0 Measured at 25 ∞C... the soil pore water or groundwater BTEX compounds 20 00 CRC Press LLC TABLE 2. 7 Highly Soluble Components of Gasoline Compound Benzene Toluene Ethylbenzene o-Xylene p-Xylene m-Xylene 2- Butene 2- Pentene Butane 1 ,2, 4-Trimethylbenzene Pentane a Percent in Gasoline by Weight 1.94 4.73 2. 0 2. 27 1. 72 5.66 0.315a 0.435a 3.83 3 .26 3.11 Average of cis- and trans- are so frequently encountered in groundwater in... uptake is suppressed by the presence of water 20 00 CRC Press LLC TABLE 2. 14 Vapor Densities and Pressures for BTEX and MTBE Compound Vapor Density (g/L) Vapor Pressure (mmHg) Temperature (∞C) Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene MTBE 3.19 3.77 4.34 4.34 4.34 4.34 — 76 22 12 6.6 8.3 8.8 24 5 20 20 30 25 25 25 25 (Chiou and Shoup, 1985; Ong and Lion, 1991a,b; Poe et al., 1988; Rhue et... in Gasoline, Diesel, and Crude Oil Effective Solubility (ppm) Compounds Gasoline Diesel California Crude Oil Benzene Toluene o-Xylene m-Xylene p-Xylene Ethylbenzene 44.39 26 .54 3 .26 8.45 3 .25 2. 87 8.83 3 .25 1.39 1.44 1. 82 0.55 0.70 1.54 0 .25 0 .26 0. 32 0.74 20 00 CRC Press LLC TABLE 2. 10 Kinematic Viscosity of Refined Products Product Gasoline Diesel Kerosene JP-4 No 1 fuel oil No 2 fuel oil No 4 fuel... spirits Soltrol Jet fuel JP-4 JP-5 JP-7 JP-8 JP-A JP-B Gasoline Naphtha (petroleum ether) 25 ∞C 15∞C 1. 028 0.969 0.917 0.907 0.898 0.883 0.878 0.885 0.8–0.88 0.874 0.866 0.865 0.849 0.840 0. 82 0.86 0.8 62 — 0.793 0.789 0.77–0.84 0.755 0.788 0.779 0.840 0.775 0.757 0. 720 0.640 — 0.974 0. 923 — 0.904 — — — — — — — 0.839 0 866 — — 0. 827 — — — — 0.844 — — — — 0. 729 — through the soil and groundwater due to density... MTBE Solubility at 0∞C (ppm) Compound Benzene (C6H6) Toluene (C6H5CH3) Ethylbenzene (C6H5CH2CH3) m-Xylene (C6H4(CH3 )2) o-Xylene (C6H4(CH3 )2) p-Xylene (C6H4(CH3 )2) MTBE ((CH3)3C(OCH3)) Solubility at 20 ∞C (ppm) — 724 197 196 1 42 — — Solubility at 25 ∞C (ppm) 1780 515 153 158 1 52 — — 1760 573 177 146–173 21 3 180 20 0 48,000 viscosity” which is the viscosity of the substance divided by its density The viscosity... compounds in the pharmaceutical literature and in soil research (Lane and Loehr, 19 92) 20 00 CRC Press LLC TABLE 2. 12 Residual Saturation (mg/kg) of Refined Products in Soil Soil Type Coarse gravel Gravel to coarse sand Coarse to medium sand Medium to fine sand Fine sand to silt Gasoline No 2 Fuel Oil Lube Oil No 6 Fuel Oil — — — 20 00 — 800 1600 28 00 4800 8000 1600 320 0 5600 9600 16,000 — — — 60,000 — Another... g/m2 day–1, g/m3 year–1, mg/day per bacterial cell, percent of oil removed after a known number of days or weeks, or g/m3 day–1 A 20 00 CRC Press LLC TABLE 2. 11 Biodegradation Half-Lives of BTEX and PAH Compounds Biodegradation Half-Life (hr)a Compound Benzene (C6H6) Toluene (C6H5CH3) Ethylbenzene (C6H5C2H5) o-, m-, p-Xylene (C6H4(CH3 )2) Acenaphthene (C12H10) Anthracene (C14H10) Benzo(a)pyrene (C20H 12) ... concern with gasoline releases Analytical methods used to test for MTBE are EPA Methods 8 020 and 824 0/ 826 0 20 00 CRC Press LLC For drinking waters, EPA Method 524 is used Another method is a modified ASTM Method D4815, which tests samples for methanol, ethanol, ethyl-tertiary-butyl-ether (ETBE), and tertiary-methyl-ether (TAME), as well as MTBE The separation of MTBE ((CH3)3C(OCH3)) from the gasoline... Chiou, C and T Shoup, 1985 Soil sorption of organic vapors and effects of humidity on sorptive mechanism and capacity, Environmental Science and Technology, 19:1196– 120 0 Cirvellol, J., Radovsky, A., Heath, J., Farnell, D., and C Lindamood, 1995 Toxicity and carcinogenicity of t-butyl alcohol in rats and mice following chronic exposure in drinking water, Toxicology and Industrial Health, 11 (2) :151–166 . 3 .25 1.54 o-Xylene 3 .26 1.39 0 .25 m-Xylene 8.45 1.44 0 .26 p-Xylene 3 .25 1. 82 0. 32 Ethylbenzene 2. 87 0.55 0.74 TABLE 2. 8 Solubility of BTEX Compounds and MTBE Solubility at 0∞C Solubility at 20 ∞C. “kinematic TABLE 2. 7 Highly Soluble Components of Gasoline Compound Percent in Gasoline by Weight Benzene 1.94 Toluene 4.73 Ethylbenzene 2. 0 o-Xylene 2. 27 p-Xylene 1. 72 m-Xylene 5.66 2- Butene 0.315 a 2- Pentene. reactions, and cosolva- tion. FIGURE 2. 6 Chromatograms of JP-4 and JP-5. (From Bruya, J., Chromatograms, Friedman and Bruya, Seattle, WA, 1999. With permission.) 20 00 CRC Press LLC 2. 4.1 HENRY’S