What is Diesel Fuel

W. Addy Majewski, Hannu Jääskeläinen

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Abstract: Diesel fuel is a mixture of hydrocarbons obtained by distillation of crude oil. The important properties which are used to characterize diesel fuel include cetane number (or cetane index), fuel volatility, density, viscosity, cold behavior, and sulfur content. Diesel fuel specifications differ for various fuel grades and in different countries.

Crude Oil Refining

The beginnings of the petrochemical industry date back to the 1850s. The first modern oil refineries were built by Ignacy Łukasiewicz near Jasło, Poland (then under Austrian rule) in 1854–56 [3410]. The refined products were used in Łukasiewicz’s kerosene lamp, as well as in artificial asphalt, machine oil and lubricants. A few years later, in 1859, crude oil was discovered in Pennsylvania in the United States. The first product refined from crude in Pennsylvania was also kerosene, used as lamp oil [1149].

Since only a fraction of the crude could be refined into kerosene, the early refiners were left with quantities of petroleum by-products. These petroleum by-products attracted the attention of Rudolf Diesel, the inventor of compression ignition reciprocating engine. Diesel—whose first engine concept was designed to use coal dust as the fuel—recognized that liquid petroleum products might be better fuels than coal. The engine was re-designed for operation with liquid fuels, resulting in a successful prototype in 1895. Both the engine and the fuel still bear the name of Diesel.

Diesel fuel is a mixture of hydrocarbons—with boiling points in the range of 150 to 380°C—which are obtained from petroleum. Petroleum crude oils are composed of hydrocarbons of three major classes: (1) paraffinic, (2) naphthenic (or cycloparaffinic), and (3) aromatic hydrocarbons. Unsaturated hydrocarbons (olefins) rarely occur in the crude. It should be noted that the terms ‘paraffinic’ and ‘naphthenic’ sound obsolescent; we use them because they are still common in the petrochemical industry. In modern chemistry, the respective groups of hydrocarbons are called alkanes and cycloalkanes.

The composition of the crude can vary from thin light-colored brownish or greenish crude oils of low density, to thick and black oils resembling melted tar. The thin, low density oils are called “high-gravity” crude oils, and the thick high density ones, “low-gravity” crude oils. This convention, rather confusing to those outside the petroleum industry, is explained by the use of “API gravity” which is a fuel property inversely proportional to its density, Equation (5).

In the refining process, the crude oil is converted into transportation fuels—gasoline, jet fuel, and diesel fuel—and other petroleum products, such as liquefied petroleum gas (LPG), heating fuel, lubricating oil, wax, and asphalt. High-gravity crude oils contain more of the lighter products needed for the production of transportation fuels, and generally have lower sulfur content. Modern refining processes can also convert low-gravity crude oils into lighter products, at an added expense of more complex processing equipment, more processing steps, and more energy.

Modern refining processes can be classified into three basic categories:

A schematic of modern refinery with diesel streams highlighted is shown in Figure 1 [1149]. In the primary distillation column, operating under atmospheric pressure, the crude oil feedstock is separated into a number of streams of increasingly higher boiling point, which are called straight-run products (e.g., straight-run diesel). The material that is too heavy to vaporize in atmospheric distillation is removed from the bottom of the column (so called “atmospheric bottoms”). In most refineries, the atmospheric bottoms are further fractionated by a second distillation carried out under vacuum.

Figure 1. Diesel streams in modern refinery

AGO - atmospheric gas oil; VGO - vacuum gas oil; HCO - heavy cycle oil

(Courtesy of Chevron)

The quantity and quality of the streams drawn off from distillation depends on the chemical composition of the crude oil. Crude oils also yield proportions of gasoline, diesel, residual fuel oil, and other products which are usually different from the product demand patterns in particular markets. The only way to balance the refinery production pattern with market demands is through downstream conversion processes. In these conversion processes large hydrocarbon molecules are broken into smaller ones by application of heat, pressure, or catalysts. Refineries use thermal cracking (visbreaking and coking), catalytic cracking, and hydrocracking (also utilizing catalyst, but carried out under a high pressure of hydrogen) to increase the yield of desired products by cracking unwanted heavy fractions. The final products are obtained by blending conversion products (crack components) with the primary distillation streams.

Both blended and straight-run products may require a varying degree of upgrading, to reduce the content of sulfur, nitrogen, and other compounds. A range of processes called hydroprocessing use hydrogen with an appropriate catalyst to upgrade refinery streams. Hydroprocessing can vary from mild condition hydrofinishing that removes reactive compounds like olefins and some sulfur and nitrogen compounds, to more severe condition hydrotreating that saturates aromatic rings and removes almost all sulfur and nitrogen compounds.

As apparent from Figure 1, diesel fuels used in road transportation are distillate fuels, i.e., they do not contain (uncracked) residuum fractions. Petroleum residuum materials are contained in heating oils, as well as in marine fuels (also known as bunker fuels). Those products usually have largely different properties from distillate diesel fuels.