By Scott A. Stout, Allen D. Uhler, Kevin J. McCarthy,
Stephen Emsbo-Mattingly, and Gregory S. Douglas

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 major refining processes and the general influences these have on production of petroleum products and intermediates. Part 2 of this series focused on the blending practices used in the production of automotive gasolines and the effects these have on 'fingerprinting' gasolines. In this installment, we focus on the refining
practices used in the production of distillate fuels. Particular attention
is given to those features that can be useful in environmental forensic
investigations in which the type(s) and source(s) of distillate fuels are in
question.

What are Distillate Fuels?

Distillate fuels refers to a category of fuels, largely classified depending
upon their intended use. They include civilian and military jet engine
fuels, on-road diesel (truck and bus), off-road diesel (rail, heavy
equipment, and farm machinery), marine diesel engine fuels, non-aviation gas turbine fuels, and domestic and commercial heating fuels (Table 1). As their name implies, the production of distillate fuels involves vaporizing
and re-condensing, which distinguishes them from the higher boiling,
residual fuels (e.g., fuel oil #6). Volumetrically, on-road diesel fuel #2
and civilian jet fuel (Jet A) comprise the bulk of distillate fuel produced
at U.S. refineries.

With minor exceptions, distillate fuels generally boil within the range of
approximately 100oC to 400oC, which roughly corresponds to a carbon range of C7 to C25. There is considerable 'overlap' in the chemical and physical properties of some distillate fuel types. For example, the general
characteristics for diesel fuel #2 (on- and off-road), gas turbine fuel #2,
and fuel oil #2 are generally comparable (see Table 2 and description
below). Regardless of sharing general properties, the specific
characteristics within a particular fuel type will depend upon (1) the
specific "recipe" by which it was refined and/or blended (e.g., hydrotreated versus straight-run), (2) the nature of the crude oil feedstock (e.g., sweet versus sour crude), and (3) the intended market (e.g., on-road versus off-road grade diesel fuel). Each of these factors can introduce considerable variability in the detailed molecular composition of distillate fuels. This variability provides an opportunity for the environmental forensic investigator to unravel issues involving recognizing the type(s), and thereby perhaps the source(s), of distillate fuels in the environment.

Distillate Fuel Specifications Relevant to "Fingerprinting"

Although numerous ASTM and military specifications exist for all types of
distillate fuels (Table 1), in practice these are intended to assure that a
particular distillate fuel (1) can be conveniently and safely handled during
shipping and storage, (2) performs well under the intended operating
conditions, and (3) minimizes maintenance due to excess engine deposits or wear. The proxies for these properties that are listed in each product's specifications. These proxies include various bulk physical and chemical properties, e.g., volatility, ignition quality (i.e., cetane number),
stability, viscosity, color, ash content, water and sediment content, etc.
The specified values for these generally present minima or maxima values
that permit convenient and safe use of a given fuel. Some distillate
fuels' ASTM specifications are nearly identical, for example, diesel fuels
#2 and fuel oil #2 (Table 2), which allows, for example, on-road diesel fuel
#2 to be "re-branded" and safely sold for use in off-road diesel engines and in home heating furnaces. While the existing specifications provide
refiners with considerable flexibility in producing and marketing most
distillate fuels, in practice it is customer satisfaction that dictates
quality, and it is common for large volume costumers to provide refiners
with more stringent specifications (than may be called for under ASTM) for the fuel(s) they wish to purchase. The combination of refiner flexibility
and customer-specific demands introduces considerable variability in the
detailed chemical features within a given distillate fuel type. Thus, as
mentioned above, this variability provides forensic investigators an
opportunity to recognize and distinguish different varieties of a given
distillate fuel type.

An important specification for distillate fuels is their sulfur content
(Table 2). Sulfur in distillate fuels has always been a concern due to
acidity it produces during combustion, the detrimental effects (corrosion,
wear, and deposit build-up) this has on engine and furnace parts (Gruse,
1967), and the implications for deleterious air quality impacts. As a
result, sulfur content of most distillate fuels has been long specified.
The first fuel U.S. specification for diesel fuel #2, dating from 1922,
required <1.5 %vol sulfur (< 15,000 ppm; Gruse, 1967). However, it was
quickly learned that the higher the sulfur content, the greater were the
maintenance problems encountered in diesel engines. Thus, in practice most historic diesel fuels contained < 5000 ppm sulfur.

In 1993, due to concerns surrounding air emission (not engine maintenance), the EPA required that "low sulfur", on-road varieties of diesel fuel contain < 500 ppm sulfur (Table 2). (In 1993, California required off-road (non-railroad) diesels to meet the same 500 ppm maximum as on-road diesels.) Prior to 1993, on-road diesel fuels #2 contained an average of 2,500 ppm sulfur (EPA, 2000), i.e., five times higher than current limit. The sulfur content of off-road diesel fuels was historically higher than in on-road diesel fuels (NIPER, 1998). For example, Gruse (1967) reports the average sulfur contents in off-road and on-road diesel fuels in 1965 were 2,000 to3,800 ppm and 300 to 2,400 ppm, respectively. This difference was (and is) because off-road diesel engines (rail or farm and heavy equipment) can tolerate higher sulfur fuels since they operate at higher power outputs, higher temperatures, and at relatively constant speeds and load conditions (as compared to smaller on-road diesel engines; Jewitt et al., 1993). An even higher sulfur content can be tolerated in marine diesel engines, allowing marine diesel fuel (grade A) to contain up to 15,000 ppm sulfur (Table 2). While modern home heating oils contain less sulfur (Table 2), home heating oils historically contained even higher sulfur than off-road or marine diesel. For example, in the mid-1960's home heating oils could contain up to 16,000 ppm (Schmidt, 1969).

Over the past couple years, the continued concern over air emissions from land-based diesel engines has led to a mandate for even more stringent sulfur specifications for on-road diesel fuels in the future. The EPA has proposed a rule that would require refiners to further reduce the sulfur maximum in 80% of the on-road diesel fuels sold from the current maximum, 500 ppm, to 15 ppm (0.0015 %vol) by June 1, 2006. (The remaining 20% of the on-road diesel would need to meet the 15 ppm limit by 2010.) Refiners contend this new rule is beyond EPA's jurisdiction, and the matter is currently being litigated and debated.

Relevant Refining Practices in the Production of Distillate Fuels
Each of the refineries in the United States are configured somewhat
differently to work with different crude oil feedstocks, and optimized to
produce a particular suite of refined products. Each refinery takes a
particular slate of crude oil, which may change over time, and makes
marketable petroleum products, while attempting to maximize margins. In
the case of distillate fuels, historic practice was to distill
"straight-run" products directly from the parent crude oil feedstock,
without further processing. This practice limits refiners' options in
producing distillate fuels that meet modern specifications. However, given
the availability of refining intermediate streams, most modern refineries
blend cracked intermediated products (e.g., light- and mid-cut cycle oils,
visbreaker or coker gas oil, or hydrocrackate) with straight-run distillate
products (e.g., light and heavy straight run (virgin) distillates) in order
to produce their distillate fuels (Jewitt et al. 1993).

Figure 1 shows the total ion chromatograms for three different distillate
blending stocks in use at a single refinery. Each of the blending stocks
contains a different range of hydrocarbons, though all are within the
distillate range. The boiling distributions (i.e., the fingerprint) of the
distillate fuels produced from these blending stocks will depend upon the
blending "recipe". A particular recipe will be reflected in the n-alkane
(or other hydrocarbon) profile of the final, blended fuel. For example,
off-road diesel fuels are often blended to contain a greater proportion of
higher-boiling distillate hydrocarbons (Gary and Handwerk, 1984). This
blending practice, and the effect that it will have on the resulting
chemical fingerprint, could be useful in forensic investigations requiring
recognition of different varieties of a particular distillate fuel type.

There are currently 124 refineries in the U.S. that produce low sulfur
diesel fuels (EPA, 2000). The lower sulfur requirements of modern on-road diesel fuel #2 (500 ppm; Table 2) requires these refineries to employ some form of distillate desulfurization in order to produce on-road diesel fuel. Most commonly, this requires the "hydro-treatment" of distillate blending stocks, which reduce sulfur-containing compounds by replacing sulfur with hydrogen. However, some sulfur can beneficially act as antioxidants, which can improve the handling of distillate fuels. Thus, hydrotreated distillate fuels generally require refiners to add antioxidants (e.g., hindered phenols) to improve their handling. These additional refining steps add considerable cost to the production of low sulfur, on-road diesel fuel. As a result, marine and off-road diesel fuel #2 typically have (and have had) higher sulfur contents than on-road diesel fuels.

We have found a useful proxy for the sulfur content in distillate fuels is
the relative concentration of alkylated dibenzothiophenes, as reflected in
the ratio of alkylated dibenzothiophenes to alkylated phenanthrenes (e.g.,
Douglas et al. 1996). Ratios between the two-carbon (C2) and three-carbon (C3) alkylated derivatives of these compounds-D2/P2 and D3/P3- can therefore help distinguish distillate fuels subject to different degrees of desulfurization. (These ratios also reflect the original sulfur content of
the parent crude oil(s)). Figure 2 shows a cross-plot of these indices for
a suite of soils from a terminal site impacted by distinct distillate fuels.
The diesel fuels were sufficiently weathered so that distinctions using
GC/FID fingerprints were inconclusive. However, the PAH data revealed that at least two distinct types of diesel fuel could be recognized in the site's soils. Diesel B contained more dibenzothiophenes (i.e. sulfur) than Diesel. A. A third type (C) or mixture of the two was also evident.

Relevant Effects of Crude Oil Feedstock on Distillate Fuels

While the refining processes are certainly important, some properties of
distillate fuels are dependent upon the parent crude oil feedstock.
Features "inherited" from the parent crude oil feedstock can include ratios
between isoprenoids (e.g., pristane and phytane) or petroleum biomarkers. The latter are particularly useful due to their specificity and resistance to weathering and most refining steps (Peters et al. 1992). Biomarkers within the distillate range include bicyclic hydrocarbons known as sesquiterpanes. These hydrocarbons are 'low boiling' biomarkers that can provide diagnostic information about the source of distillate fuels (Stout et al. 1999). For example, Figure 3 shows the partial total ion chromatogram (TIC) and m/z 123 mass chromatograms for a fresh diesel fuel #2. Normal alkanes dominate the TIC, but ten sesquiterpanes are revealed in the m/z 123 mass chromatogram (tentatively identified after (Noble et al. 1986). These compounds are "inherited" from the parent crude oil feedstock used in the production of this diesel fuel. These bicyclic compounds are relatively resistant to weathering (as compared to the n-alkanes and isoprenoids) and can be useful in recognizing distinct distillate fuel types in the environment.

Relevant Effects of Additives on Distillate Fuels

Additives to distillate fuels can include cetane improvers, pour point
depressants, wax crystal modifiers, anti-smoke additives, antioxidants,
metal deactivators, anti-haze additives, biocides, corrosion inhibitors, and
dyes (Henry, 1988; Owen and Coley, 1990). In the case of dyes, red dyes are currently used in distinguishing diesel fuels for on-road versus
off-road purposes (ASTM, 1997), which can provide forensic investigators
with an opportunity to distinguish these very similar fuel types. However,
the dye distinction between on-road and off-road diesel fuel has not always been the case. Since 1993, the EPA has required that off-road (i.e., high sulfur) diesel fuels be dyed in order to distinguish them from on-road (i.e., low sulfur) diesel fuels, which are un-dyed/clear. Initially,
starting in October 1993, off-road (high sulfur) diesel fuels #2 were dyed
blue using 1,4-dialkyl amino anthraquinone (40 C.F.R. section 80.29).
However, refiners contended this practice might lead to confusion between off-road diesel fuels #2 and aviation gasolines, which are also dyed blue. As a result, on October 1, 1994 the original blue-dye requirement was changed to require that all off-road diesel fuels be dyed red (40 C.F.R. section 80.29). Thus, only since October 1994 has off-road diesel fuel has been dyed red. Prior to 1993 there were no specifications requiring or prohibiting refiners from dyeing diesel fuels #2 of any type, on-road or off-road, any color they wished. While this was sometimes done for marketing purposes, the dyeing of diesel fuels #2 was not common since additional costs were incurred.

Summary

Distillate fuels represent a broad range of petroleum products used in both commercial and military facilities, on both land and sea. The refining of distillate fuels involves distillation, (hydro-) treatment, custom blending,
and use of additives to meet ASTM or more stringent, customer-based
specifications. The variety of crude oil feedstocks in use, the variety of
distillate fuels produced, and the "recipes" by which refiners produce these fuels, introduces variability into the distillate market. This, in turn,
provides the forensic investigator with opportunity to recognize distinct
fuel 'types', which can help unravel issues of fuel 'source'. Of course,
chemical fingerprinting is only one part of a forensic investigation, since
it alone can be confounded by fungible pipelines, downgrading (re-branding) of products, etc. Ultimately, the ability to defensibly distinguish a distillate fuel's particular source in the environment requires knowledge of crude oil chemistry, distillate fuel refining and distribution practices -
past and present, regulatory history, as well as local geologic and
hydrologic conditions.

References

American Society for Testing and Materials (1997). Standard specification for diesel fuel oils. D975-97.
EPA (2000). Fuel standard feasibility. In: Heavy Duty Standards/Diesel Fuel
RIA EPA420-R-00-026, 122 p.
Douglas, G.S., Bence, A.E., Prince, R.C., McMillen, S.J., and Butler, E.L.
(1996). Environmental stability of selected petroleum hydrocarbon source and
weathering ratios. Environ. Sci. Technol. 30, 2332-2339.
Gary, J.H. and Handwerk, G.E. (1984), "Petroleum Refining" (Second
Edition), 414 pp. Marcel Dekker, Inc., New York, NY
Gruse, W.A. (1967) Motor Fuels, Performance and Testing. Reinhold
Publishing Corp. New York, NY., 280 pp.
Henry, Cyrus P. (1988) Additives for middle distillate and kerosine fuels In
"Distillate Fuel: Contamination, Storage and Handling. ASTM STP 1005" (eds.
H.L. Chesneau and M.M. Dorris), pp. 105-113. American Society for Testing
and Materials, Philadelphia, PA
Jewitt, C. H., Westbrook, S. R., Ripley, D. L., and Thornton, R. H. (1993)
Fuels for land and marine diesel engines and for nonaviation gas turbines.
In "ASTM Manual on Significance of Tests for Petroleum Products, 6th
Edition" (ed. G.V. Dyroff) American Society for Testing and Materials,
Philadelphia, PA
NIPER (1998). Diesel fuel oils, 1998. National Institute of Petroleum and
Energy Research, BDM Technologies Publ. NIPER-207 PPS-98/5,
Noble, R.A., Alexander, R., Kagi, R.I., and Knox, J. (1986). Identification
of some diterpenoid hydrocarbons in petroleum. Org. Geochem. 10, 825-829
Owen, K. and Coley, T. (1990). Diesel fuel additives. In: Automotive Fuels
Handbook, SAE International 423-442
Peters, K.E., Scheuerman, G.L., Lee, C.Y., Moldowan, J.M., Reynolds, R.N.,
and Pena, M.M. (1992). Effects of refinery processes on biological markers.
Energy Fuels 6, 560-577
Schmidt, P. (1969), "Fuel Oil Manuel" (Third Edition), 263 pp. Industrial
Press, Inc., New York, NY
Song, C. (2000). Introduction to chemistry of diesel fuels. In, Chemistry of
Diesel Fuels Taylor & Francis, New York,
Stout, S.A., McCarthy, K.J., Seavey, J.A., and Uhler, A.D. (1999)
Application of low boiling biomarkers in assessing liability for fugitive
middle distillate petroleum products. In Ninth Annual West Coast Conference,
Contaminated Soils and Water. Oxnard, CA.

Top