Petroleum weathering

Of particnlar significance in the study of petroleum weathering are the biomarker molecules (e.g., pristane, phytane, the hopanes and steranes). Historically, the biomarkers have been employed as crude oil signatures in prospecting and characterization. More recently, such molecules have also been employed in the environmental field, both for the determination of pollutant source and estimation of the degree of weathering. 

Spectra of Petroleum Weathered Naturally and Under Simulated Conditions, Environ. Sci. Technol. 1976,10(8), 777. 

R. E. Jordan and J. R. Payne, Fate and Weathering of Petroleum Spills in the Marine Environment, Ann Arbor Science Pubhcations, Ann Arbor, Mich., 1980. 

Biodiesel does not present any special safety concerns. Pure biodiesel or biodiesel and petroleum diesel blends have a higher flash point than conventional diesel, making them safer to store and handle. Problems can occur with biodiesels in cold weather due to their high viscosity. Biodiesel has a higher degree of unsaturation in the fuel, which can make it vulnerable to oxidation during storage. 

In contrast to infrared spectrometry there is no decrease in relative sensitivity in the lower energy region of the spectrum, and since no solvent is required, no part of the spectrum contains solvent absorptions. Oil samples contaminated with sand, sediment, and other solid substances have been analysed directly, after being placed between 0.5 mm 23-reflection crystals. Crude oils, which were relatively uncontaminated and needed less sensitivity, were smeared on a 2 mm 5-reflection crystal. The technique has been used to differentiate between crude oils from natural marine seepage, and accidental leaks from a drilling platform. The technique overcomes some of the faults of infrared spectroscopy, but is still affected by weathering and contamination of samples by other organic matter. The absorption bands shown in Table 9.1 are important in petroleum product identification. 

Douglas, G., Bence, A., Prince, R., McMillen, S., and Butler, E., 1996, Environmental Stability of Selected Petroleum Hydrocarbon Source and Weathering Ratios Environmental Science and Technology, Vol. 30, pp. 2332-2339. 

Chromatographic Evidence Sample chromatograms often contain evidence that natural attenuation has occurred. If chromatograms of fresh petroleum that was released can be obtained, then a solid comparison can be made between fresh and weathered samples relatively soon after being released in many circumstances. 

Recovery of LNAPL via trenches was specified in the ROD but found to be unsuccessful due to its discontinuous and relatively thin apparent thickness of less than 1 in. A backhoe test pit program revealed the hydrocarbons to be distributed over a distinct 3-ft-thick smear zone. Observation of the hydrocarbons in the smear zone indicated significant weathering may have occurred as evidenced by a silver-gray staining of petroleum on the sand and gravel. 

Residual products (No. 6 fuel oil, bunker C oil) these products have little (usually, no) ability to evaporate. When spilled, persistent surface and intertidal area contamination is likely with long-term contamination of the sediment. The products are very viscous to semisolid and often become less viscous when warmed. They weather (oxidize) slowly and may form tar balls that can sink in waterways (depending on product density and water density). They are highly adhesive to soil. Heavy oil, a viscous petroleum, and bitumen from tar sand deposits also come into this category of contaminant. 

Evaporative processes are very important in the weathering of volatile petroleum products and may be the dominant weathering process for gasoline. Automotive gasoline, aviation gasoline, and some grades of jet fuel (e.g., JP-4) contain 20 to 99% highly volatile constituents (i.e., constituents with fewer than nine carbon atoms). 

The most common method for GC/MS analysis of semivolatile compounds (EPA SW-846 8270) includes 16 polycyclic aromatic compounds, some of which commonly occur in middle distillate to heavy petroleum products. The method also quantifies phenols and cresols, compounds that are not hydrocarbons but may occur in petroleum products. Phenols and cresols are more likely found in crude oils and weathered petroleum products. 

Rather than quantifying a complex total petroleum hydrocarbon mixture as a single number, petroleum hydrocarbon fraction methods break the mixture into discrete hydrocarbon fractions, thus providing data that can be used in a risk assessment and in characterizing product type and compositional changes such as may occur during weathering (oxidation). The fractionation methods can be used to measure both volatile and extractable hydrocarbons. 

In contrast to traditional methods for total petroleum hydrocarbons that report a single concentration number for complex mixtures, the fractionation methods report separate concentrations for discrete aliphatic and aromatic fractions. The petroleum fraction methods available are GC-based and are thus sensitive to a broad range of hydrocarbons. Identification and quantification of aliphatic and aromatic fractions allows one to identify petroleum products and evaluate the extent of product weathering. These fraction data also can be used in risk assessment. 

The total petroleum hydrocarbons represents a summation of all the hydrocarbon compounds that may be present (and detected) in a soil sample. Because of differences in product composition between, for example, gasoline and diesel, or fresh versus weathered fuels, the types of compounds present at one site may be completely different from those present at another. 

Accordingly, the total petroleum hydrocarbons at a gasoline spill site will be comprised of mostly Cs to Cu compounds, while total petroleum hydrocarbons at an older site where the fuel has weathered will likely measure mostly Cg to Cn compounds. Because of this inherent variability in the method and the analyte, it is currently not possible to directly relate potential enviromnental or health risks with concentrations of total petroleum hydrocarbons. The relative mobility or toxicity of contaminants represented by total petroleum hydrocarbons analyses at one site may be completely different from that of another site (e.g., Ce to Cn compared to Cio to C25). There is no easy way to determine if total petroleum hydrocarbons from the former site will represent the same level of risk as an equal measure of the total petroleum hydrocarbons from the latter. For these reasons it is clear that the total petroleum hydrocarbons value offers limited benefits as an indicator measure for cleanup criteria. Its current widespread use as a soil cleanup criterion is a function of a lack of understanding of its proper application and... 

The density of crude oil is on the order of 0.85 g/cm, so if the sea surface is calm, an oil spill will initially form a slick. The slick is subject to physical processes, such as advection and turbulence, causing it to move vertically and/or horizontally. Advection tends to lead to dispersal or, if land is nearby, shoreline stranding. Turbulence promotes the formation of emulsions, called chocolate mousse, which can be transformed via weathering into tarballs. The lower-molecular-weight compounds tend to evaporate or dissolve. Some fractions of petroleum have solubilities in seawater on the order of tens of milligrams per liter. Some are also photochemically oxidized. 

Fines from petroleum coke are too small to use alone for domestic heating. But petroleum coke fines have been briqueted successfully, and the briquets used as domestic fuel (21, 88). In one instance, briquets made with 5 to 10% of refinery acid sludge as the binder were roasted to 1100° F. The resulting briquets were clean, hard, strong, and smokeless and could be stored or shipped in any weather. Uncalcined briquets tend to soften and deform in hot weather (16). 

Bitumen describes a black or dark brown masticlike material that is thermoplastic in nature and softens upon heating. The sources of bitumen are petroleum or coal deposits. The natural product is commonly called gilsonite or pitch, a mineral formed by an old weathered petroleum flow at the surface of the earth that has left behind the larger molecules from the petroleum. A principal source in the past has been Lake Trinidad, a 445,000 m2 deposit on the island of Trinidad. Bitumen from petroleum or crude oil is called asphalt (qv). It is the material left behind after all the valuable compounds, eg, gasolines, have been distilled out of the cmde oil. The amount and quality of asphalt is dependent on the source of the crude oil used in the refining process. Some cmde oils have a higher content of asphaltic bitumen left after the distillation process. Bitumen from coal is coal-tar pitch. It remains after the valuable coal oils and tars have been distilled out of the coal tars produced by distractive distillation. Most industrial applications for bitumen products use asphalt or coal-tar pitch because the supply is more uniform and plentiful. 

Raw data gathered from seismic surveys must be processed to compensate for and to remove a variety of distortions unwanted noises created by weathered near-surface rocks, normal time delays, and echoing by rebounding acoustic waves—to provide the clearest possible image of the strata below. Computers can restore these distortions in a fraction of the time that was formerly required to adjust the data painstakingly by hand. Advanced techniques not only permit presentations in three dimensions, but also in color, and to create contour maps and models of subterranean features. However, even with the use of sophisticated tools, there remains a large measure of uncertainty. History has shown repeatedly that a prospective area rejected by one petroleum firm has been accepted by another and proved to be successful. 

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