Introduction
The operations at modern refineries impart certain chemical characteristics to the petroleum products produced. Part 1 of this series provided an overview of the variety of modern refining processes and the general influences these have on production of petroleum products and intermediates. Under most circumstances, modern refiners adjust feedstock and process streams in order to optimized their production capacity for one product, automotive gasoline. In fact, over the last fifty years refiners have increasingly squeezed more gasoline from crude oil, and now utilize more than 50% of a barrel of crude oil in the production of automotive gasoline (Figure 1). In this article, the specific refining influences that affect gasoline composition are reviewed with particular attention given to those features that can be useful in environmental forensic investigations in which gasoline sources are in question.

Gasoline Specifications Relevant to “Fingerprinting”
Automotive gasoline’s commercial success over the past 100 years stems largely from the early and continuing cooperation between petroleum refiners and automobile manufacturers. The American Society for Testing and Materials (ASTM) provides a forum for this cooperation, and has ultimately led to gasoline specifications adopted by refiners, gasoline marketers, automakers, automotive engineers, and, more recently, regulatory agencies such as the U.S. EPA. ASTM’s current standard for automotive gasoline, D 4814 – Standard Specification for Automotive Spark-Ignition Engine Fuel, refers to a variety of appropriate analytical methods and mandated limits for vapor pressure/volatility (seasonally and geographically variable), sulfur content, water tolerance, oxidation stability, existent gum content, lead content, and copper strip test. Additional regulatory limits (Federal and State) on properties not specifically addressed in ASTM D 4814, e.g., oxygen and phosphorous content, are described in an appendix (X3) to the standard. Notably, the octane of automotive gasolines is not specified by ASTM D 4814. This ‘performance-based’ feature is left up to individual refiners and marketers.

Two items from this discussion are relevant to practitioners of environmental forensics, namely (1) the flexibility that refiners are given to achieve the volatility/vapor pressure, sulfur content, oxygen content specifications and (2) the various means by which refiners achieve octane targets. (These two are not unrelated since changes in composition will potentially affect octane.) The relevance stems from the fact that different refiners produce gasoline in different ways – i.e., not all gasoline is created equal. Such chemical differences among automotive gasolines, of course, may be crucial in environmental forensic investigations where there is the need to defensibly recognize the presence of distinct gasoline types.

Gasoline Blending
The ASTM-mandated and ‘performance-based’ requirements of gasolines are achieved by different refiners in different manners. While some final blending (e.g., addition of detergent additive packages) may occur during product distribution, the majority of a gasoline’s properties are achieved through the blending that occurs within the petroleum refinery. The manner in which an individual refinery is configured and operated, which can even include purchasing some blending stocks from nearby refineries, will dictate how the requirements are met – and this may change over time. Two refineries that are configured or operated differently will produce gasolines with different chemical signatures, yet both refineries’ products will meet the same ASTM and octane requirements.

Automotive gasoline is blended from a variety of intermediate blending stocks produced at different units in the refinery. Each blending stock is comprised of a unique distribution of compounds, mostly hydrocarbons, which when mixed properly achieve the ASTM-mandated and performance-based requirements for commercial gasolines. Figure 2 shows the so-called ‘PIANO’ (C5 to C12 Paraffins, Isoparaffins, Aromatics, Naphthenes, and Olefins) distributions of compounds in two gasoline blending stocks, namely, light straight run gasoline (LSRG) and reformate. The chemical differences between these blending stocks are obvious. The reformate is dominantly comprised of aromatic compounds, including BTEX and a variety of C3-alkyl-benzenes whereas the LSRG is dominated by lower boiling, aliphatic (paraffins and isoparaffins) compounds.

Table 1 shows an inventory of 14 gasoline blending stocks available at a single complex refinery. Some relevant chemical and physical features for these blending stocks are also provided. It is notable that multiple units produce blending stocks of a similar type. For example, this refinery has three reformer units that produce reformate with slightly different properties. The overall variety of different blending stocks (n=14), and the duplication of certain blending stocks, provides this refiner with a multitude of options in producing a gasoline that meets ASTM and performance-based requirements.

The ‘recipe’ for blending a specific gasoline grade at any given refinery depends upon several factors including, (1) their inventories of the various gasoline blending stocks, (2) the operating status of the various refining units, (3) the specific regulatory requirements for the intended marketing location, and, of course, (4) maximizing the profit margin. Modern refiners rely upon computer assistance in developing daily blending recipes from the available inventory that will meet the gasoline requirements. In-stream monitors provide continuous feedback on a product’s properties so that blending can be automated and self-adjusting. Consequently, generalizations regarding the chemical composition of ‘typical’ automotive gasolines can be oversimplified and dangerous for the forensic investigator. However, as noted above, it is this variety that exists among gasolines that provide the forensic investigator with an opportunity to unravel forensic issues.

Blending for Octane
Gasolines sold in the U.S. are normally available in three octane levels, or grades, namely, regular, mid-grade, and premium. In most parts of the country, these grades have corresponding octane ratings of 87, 89, and 91 to 94, respectively. (Rocky Mountain states typically have a lower octane scale due to altitude-related performance of older engines.) Because of the blending differences described above, gasolines of a similar grade (octane) can exhibit significant compositional differences. This is exemplified in Figure 3, which shows the normalized PIANO distribution for two premium reformulated gasolines (RFGs) sold in the mid-Atlantic region (an ozone non-attainment area) during the winter of 1999. Both gasolines (presumably) met the Federal RFG requirements, the ASTM requirements, and the performance requirements, yet each exhibits distinct hydrocarbon distributions. It is apparent that the RFG from refiner A achieved octane primarily from the blending of MTBE (RON 115) and iso-octane (RON 100) whereas refiner B achieved octane from MTBE and toluene (RON 124). This probably reflects a difference in refining capabilities, e.g., refiner B does not employ an alkylation unit and must rely upon aromatics (toluene) to achieve the necessary octane.

Case Study
The objective of this investigation was to determine if free phase gasoline encountered under a street separating two service stations was correlated to free phase gasolines found on two adjacent service station properties. Detailed PIANO analysis was conducted on free phase product samples from each property and from beneath the street (Figure 4). Weathering had affected the samples differently; therefore, some differences were apparent. In spite of weathering differences, the gasolines recovered from each Station revealed genetic differences related to refinery blending. Station B’s gasoline contained an abundance of particular iso-paraffins, namely, 2,2,4-, 2,3,4- and 2,3,3-trimethylpentane (Fig. 4), which indicate that Refiner B blended alkylate (Table 1) into their gasolines. Station (Refiner) A apparently did not use alkylate in production of their gasoline(s). The relative absence of these iso-paraffins in the ‘Street’ indicated it was consistent with the gasoline from Station (Refiner) A.

Summary
The refining and blending of automotive gasolines is a complex process. This complexity, however, provides the knowledgeable forensic investigator with opportunity to distinguish gasolines that have been refined by different processes. Such distinctions can be helpful in certain forensic investigations where distinguishing gasoline ‘types’ may help unravel gasoline ‘sources’. Of course, chemical fingerprinting is only one part of an investigation, since it alone can be confounded by fungible pipelines, unbranded or exchanged products, etc. Ultimately, the ability to defensibly distinguish a gasoline’s particular source requires knowledge of detailed gasoline fingerprinting, refining and distribution practices of the suppliers, as well as local geologic and hydrologic conditions.

Table 1: Gasoline Blending Stocks and Selected Features Available at a Single Complex Refinery

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