The Influences of Refining on Petroleum Fingerprinting
Part 1. The Refining Process

The majority of environmental forensics investigations involved with determining the nature and source(s) of petroleum contamination at industrial site or following a petroleum spill are focused on fuels, including light and middle distillates and residual fuels. These types of petroleum products are remarkable because, while they all have been manufactured from crude oil, they have been physically and chemically altered in some manner and blended or formulated to meet a customer- or regulatory-based product specification. While some final blending (e.g., addition of additives) may occur during product distribution, the overwhelming majority of these alterations and blends occur at the petroleum refinery. The manner in which an individual refinery is configured and operated imparts a specific chemical signature upon the products it produces. Another refinery, which is configured or operated differently, can produce different chemical signatures, yet both refineries’ products will meet the same product specifications. In environmental forensics investigations surrounding fugitive fuels, these chemical features are often at heart of what’s called “petroleum fingerprinting”. This article offers the reader a brief tour of the complicated world where petroleum products are made—the refinery.

Background

The modern petroleum refinery has evolved from a simple turn-of-the-century facility whose goal was to produce fuel oils for heating and lighting (while discarding then uneconomical gasoline-range hydrocarbons), to modern operations of varying complexity focused on squeezing as much automotive gasoline out of a barrel of crude oil as feasible while still meeting this fuels’ engineering requirements for ignition and performance, and regulatory environmental constraints both on composition and end-of-the-tailpipe emissions. The more than 150 petroleum refineries in the United States are complex industrial plants where various crude oil feedstocks are utilized to produce not only gasoline, but other economically valuable distillate and residual products. The sophistication of these refineries varies, from simple to very complex. The level of complexity is defined by the various types of equipment (i.e., units) in use at the refinery. Refineries have been built (or added to) during different engineering ‘eras’, e.g. they utilize different generations or technologies to achieve similar refining goals, all the while attempting to maximize profitability. The crude oil feedstock(s) used at each of these refineries varies with geographic setting, availability and pricing of crude, and market demand for particular products. From the environmental forensic investigator’s standpoint, it is important to realize that while the basic principles of refining are common to the industry, virtually no two refineries are identically engineered or produce refined products that, at the molecular level, are the same.

Modern Refining

What follows is a brief description of the refining units used to produce the most common commercial petroleum products. Naturally, many specialty units are found in complex and very complex refineries, and these are not discussed here. Some of the units not covered in this article (e.g., visbreakers, cokers) are used in complex refineries to produce intermediate lighter distillate feedstock streams from various kinds of high viscosity residual materials left over from the basic refining process. Other units not covered include those used for the separation or production of petrochemicals from refinery streams (e.g., olefins plant), or the synthesis of specialty chemicals for on- or off-site use or marketing (e.g., MTBE unit). For an in-depth understanding of refinery operations, we recommend one of several excellent reference texts on the subject (Gary and Handwerk, 1984; Speight, 1991; Leffler, 2000).
What should be noted in the following sections is that the goal of each refinery unit is the production of distillate streams ultimately useful in the blending of commercial fuels—especially automotive gasoline (Table 1). From the environmental forensic standpoint, remember that each time a refinery alters its crude oil feedstock or intermediate products in the pursuit of distillate fuel, it is imparting a potentially useful chemical signature upon the intermediate and commercial products it manufactures.

Distillation. The distillation unit(s) is the place where crude oil is separated into various ‘cuts’ that are defined by their effective initial boiling points and final boiling points. These cuts, obtained by both atmospheric and vacuum distillation, traditionally include light gasses (C1 to C4 hydrocarbons), straight run gasoline, naphtha, kerosene, light gas oil, heavy gas oil, and residual material. Few of these distillate ‘cuts’ are used commercially straight out of the distillation unit; rather they are shipped to other units in the refinery, either for further processing or for blending of commercial fuels.
The importance of distillation in forensic investigations rests largely on the effect that it has on the boiling ranges of the petroleum products. Distillation tends to alter the profiles of compounds depending upon their relative volatility. While the effects of distillation can affect the interpretation of certain primary features of the parent crude oil feedstock, they do not reduce the utility for comparing the chemical signatures of petroleum products in environmental media to one another. Of course, the potential effects of evaporation following a release to the environment on these signatures must be considered (McCarthy et al. 1998).

Catalytic Cracking. Earlier, we mentioned that one of main goals of the modern refinery is the production of automotive gasoline. Unfortunately, only a modest percentage of even high API gravity crude oils can be distilled to straight run gasoline. As the demand for gasoline rapidly increased in the early part of the 20th century, refinery engineers developed a process to break the larger hydrocarbon molecules found in heavy gas oils and residual fuels into much smaller, distillate fuel-range hydrocarbons. Early ‘cracking’ of hydrocarbons was accomplished by simply heating residual fuels to high temperatures to promote thermal breakdown and rearrangement of large hydrocarbon molecules. Today, the modern catalytic process is mediated by a free-flowing, recyclable catalyst in a refinery’s catalytic cracking unit (CCU). If carried out in the presence of excess hydrogen, the process is referred to hydrocracking. The useful products of catalytic cracking are cat cracked gasses, cat cracked gasoline, and cat cracked light and heavy fuel oils. These cat cracked distillates are used either for fuel blending or feedstocks to other refinery units. The residual of this process, called cycle oil, is continuously recycled as feed in the CCU. The type of cracking unit, the operating conditions of the cracker, the type and age of the catalyst used, and the nature of the feedstock to the CCU can result in different compositions of hydrocarbons in the cat cracked product stream. These differences in product stream chemistry can be used as potential markers in environmental forensics investigations.

Catalytic Reforming. The thirst for high octane gasoline blending components led to the development of the catalytic reformer. In the typical reforming process, naphtha-range distillates are mingled at high temperature and pressure with specialty catalysts containing traces of platinum or palladium. From the octane-improvement standpoint, the catalytic process converts saturated ring compounds (napthenes) into (mostly mono-) aromatic hydrocarbons, and rearranges straight-chain paraffins into branched-chain isoparaffins. The resulting stream from this process is called reformate—a gasoline-range liquid enhanced with octane-boosting mono-aromatics and isoalkanes. Reformate is a key automotive blending stream at many refineries—especially those that do not have an alkylation or isomerization unit.
Since reformates are typically blended directly into gasoline the proportions between various monoaromatics and isoalkanes can provide some diagnostic information potentially related to the reforming conditions (temperature, catalyst type and age, and feedstock). In addition, preferential removal of certain compounds from the reformate stream, e.g., ethyl-benzene’s or p-xylene’s removal for production of styrene or phthalic acid, respectively, can produce distinct signatures.

Alkylation. In complex refineries, a highly valuable automotive gasoline blending stock called alkylate is produced from the catalytic reaction of low molecular weight isoalkanes and olefins. In this process, the acid-catalyzed reaction of isobutane with propylene and/or butylenes produces a liquid enriched with high octane iso-octane and/or isoheptane and related isoparaffins. Distributions among the individual isoparaffins in an alkylate can provide useful diagnostic information that is related to the conditions of alkylation (e.g., acid-type, residence time, etc.), which, of course, varies among refiners.
Alkylate is a valuable commodity for refiners both because of it’s octane-boosting capacity and, because of it’s inherent physical and chemical properties, in the flexibility it lends in the blending of finished gasolines. One of the added benefits of alkylation is that it allows refiners to blend high octane (premium) gasoline while meeting strict limits on aromatic content for automotive gasoline.

Isomerization. Isomerization of low boiling (< C6) normal hydrocarbons to saturated branched hydrocarbons was another refining development triggered by the need for higher octane gasolines, particularly during and after World War II. In this process various catalysts and reactor conditions are used to convert C4 to C6 feed streams (butanes, pentanes, and/or pentanes-hexanes) into their various isomeric equivalents, i.e., isobutane, isopentane, 2,2-dimethlybutane, 2,3-dimethylbutane, 2-methypentane, and 3-methylpentane. The specific reaction conditions during isomerization (type and age of catalyst, feed stream and rate, and temperature) will yield rather specific distributions of C5 of C5/C6 isomers, collectively known as isomerate. Like alkylate, isomerate is a useful blending stream for improving the octane rating, and controlling the vapor pressure, of automotive gasoline.
In the case of both alkylate and isomerate, monitoring the amount and relative distributions of isoalkanes in fugitive gasolines can yield information about the nature of the isomerate blending stock that may have been used in their production. Naturally, the effects of environmental weathering must be considered.

Implications for Environmental Forensics
The chemistry of distillate and residual petroleum fuels are a function of both the primary chemical features of the parent crude oil feedstock(s) used in their production, and the alterations introduced during their distillation, conversion, and blending in the refinery. This article has presented a simplified view of the refining process, and lends insight into both the significant and subtle changes that crude oil and refinery intermediates undergo during production of commercial petroleum products. Subsequent articles in this series will address specific features related to the production of and differentiation among (1) automotive gasolines and (2) middle distillate fuels. An environmental forensics investigator must understand how these refining processes can impart diagnostic features that could prove useful in characterization or comparative studies of fugitive petroleum at contaminated sites.

References

Gary, J.H. and Handwerk, G.E. (1984), "Petroleum Refining" (Second Edition), Marcel Dekker, Inc., New York, NY, 414 pp.
Leffler, W.L. (2000). “Petroleum Refining” (Third Edition), PennWell Corp., Tulsa, OK, 310 pp.
McCarthy, K.J., Uhler, A.D., and Stout, S.A. (1998). Weathering affects petroleum ID. Soil & Groundwater Cleanup, August/September Issue.
Speight, J.G. (1991), "The Chemistry and Technology of Petroleum" (Second Edition), Marcel Dekker, Inc., New York, NY, 760 pp.

Table 1: Inventory of Major Refining Categories and their Use.

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