Petroleum: Properties, Density, Distillation, Composition, Paraffins

Properties of Petroleum

Petroleum is not a single substance. There are hundreds of different crude oils with a wide range of physical and chemical properties; properties vary with location, depth, and age of the oil field. Informally, crudes are named for their source and some key characteristics; examples include Brent, West Texas Sour, West Hackberry Sweet, and Arabian Heavy.

Formally, crudes are defined by a crude assay, as described in a subsequent section. The “sweet” and “sour” deserve comment. In the old days, prospectors characterized crude oil by tasting it. Sour crudes have more sulfur, which gives them a tart taste.

Density, Distillation, and Elemental Composition

As produced, crudes contain varying amounts of dissolved gas, water, inorganic salts, and dirt. After these are removed, what remains is an exceedingly complex mixture of chemicals, mostly organic hydrocarbons containing nothing but hydrogen and carbon. The other organic molecules contain hetero-atoms—sulfur, oxy- gen, and nitrogen, and/or trace elements (Ni, V, Fe, Cu, Hg, As, etc.). Processing costs are higher for crudes with high density and large amounts of sulfur, nitrogen, and trace contaminants.

To illustrate how widely crude properties vary, Table 5 presents the density, sulfur, and nitrogen content of 21 example oils. The data from Table 5 are presented graphically   in Fig. 10. Both sulfur and nitrogen correlate inversely with API gravity, but for this particular collection, the correlations are rough, especially for sulfur. Sulfur contents range from 0.03 wt% for Tapis to 5.3 wt% for Boscan, and nitrogen contents range from nil for Tapis to 0.81 wt% for California Beta. Specific gravities range from 0.798 for Tapis to 1.014 for Athabasca. By definition, Athabasca is “extra heavy oil,” because its specific gravity is >1.0.

In other words, it sinks in water. For “heavy oil,” the specific gravity falls between 1.0 and 0.934. The dynamic viscosities of heavy oils range from about 5000 to 10,000 centipoise (cP).

Table 5 Density, sulfur, and nitrogen content of 21 crude oils

Fig. 10 Sulfur and nitrogen versus API gravity for selected crude oils

Distillation yields are an exceptionally important property of petroleum, because they indicate relative amounts of low-boiling fractions—naphtha (which can become gasoline), kerosene (which can become jet fuel), and gas oil (which can become diesel).

Table 6 Distillation yields for four selected crude oils

Markets are limited for direct use of higher boiling fractions—atmospheric residue, vacuum gas oils, and vacuum residue-so there is a large incentive to convert them into lighter products with greater value. Table 6 shows distillation data for four common crudes. Brent contains twice as much naphtha as Ratawi, and its vacuum residue content is 60% lower. Of the four, Bonny Light yields the most middle distillate and the least vacuum residue.

Distillation cutpoints for Table 6 are as follows:

Molecular Composition

More than any other element, carbon binds to itself to form straight chains, branched chains, rings, and complex three-dimensional structures. The most complex molecules are biological—proteins, carbohydrates, fats, and nucleic acids. This is significant, because petroleum was formed from the remains of ancient microorganisms— primarily plankton and algae. As they aged in sediments under elevated temperature and pressure, these biomolecules lost olefinic and hetero- atom functional groups, leaving behind hydrocarbon skeletons.

Petroleum molecules can be categorized as saturated, aromatic, and polar compounds, or as paraffins, olefins, naphthenes, aromatics, polynaphthenes, polyaromatics, naphthenoaromatics, and heteroatom compounds. Saturated hydrocarbons can be acyclic paraffins (alkanes) or cyclic paraffins (naphthenes).

Olefins are very rare in natural petroleum. They are mainly products from thermal cracking in refineries.

Fig. 11 Structures of some simple paraffins

Paraffins

Paraffins have a general formula of CnH2n+2 . The simplest paraffin is methane with a single carbon atom. Methane is the major component of natural gas. The next member in the alkane family is ethane with two carbon atoms. After that comes propane, with three carbon atoms. When the carbon number reaches 4, isomers are possible. Isomers are chemical compounds with the same molecular formula but different structures.

Normal paraffins are unbranched. No carbon atom is connected to more than two other carbon atoms. In isoparaffins, at least one carbon atom is connected to three or four other carbon atoms. Carbon atoms connected to only one other carbon, such as the end-of-chain carbons in n-paraffins, are called primary (1◦). Carbon atoms connected to two other carbons are called secondary (2◦), those connected to three other carbons are called tertiary (3◦), and those connected to four other carbons are called (quarternary (4◦.

For example, C4H10 includes normal butane (n-C4), in which all carbon atoms are primary or secondary, and isobutane (methyl propane or i-C4), in which the central carbon atom is tertiary. C5H12 can have three isomers, normal pentane, isopentane (2-methyl butane), and neopentane (2,2-dimethyl propane), as shown in Fig. 11. The central carbon in neopentane is quarternary.

The isooctane in the figure is one of several isooctanes. Its official name is 2,2,4-trimethylpentane. This molecule serves as a standard for gasoline combustion performance in spark-ignition engines. By definition, its octane number ¼ 100.

Hydrocarbon Ring Compounds (Naphthenes and Aromatics)

Figure 12 shows examples of hydrocarbon ring compounds found in petroleum. In the figure, —R groups represent alkyl chains.

Fig. 12 Example hydrocarbon ring compounds

Naphthenes are cyclic paraffins with the gen- eral chemical formula CnH2n   . Naphthene rings can comprise 5-carbon atoms (cyclopentanes) or 6-carbon atoms (cyclohexanes). The rings generally contain paraffin side chains with either normal or iso-structures. Decalins are dinap- hthenes with two fused rings.

Aromatics contain unsaturated rings. Monoaromatics   have    the    general    formula CnH2n-6 ; the ring contains three alternating (conjugated) double bonds, in which the electrons are delocalized. The delocalization provides reso- nance stabilization energy, which gives the rings stability. The simplest monoaromatic is benzene. Like naphthenes, most aromatic rings are attached to alkyl groups. Polyaromatics con- tain two or more rings; usually the rings are condensed.

Naphthenoaromatics contain both aromatic and naphthene rings. Usually the rings are fused. They are found naturally in naphthenic crudes, and they generated by partial saturation of polyaromatics in certain refining processes.

Heteroatom Compounds

Heteroatom compounds contain sulfur, nitrogen, oxygen, and trace elements. Examples are shown in Fig. 13.

Sulfur is found primarily as H2S, mercaptans, sulfides, disulfides, thiophenes, benzothiophenes, and polybenzothiophenes. It also is found in ring compounds containing other heteroatoms. Azathiophenes, for example, contain both nitro- gen and sulfur.

Fig. 13 Example heteroatom compounds

Nitrogen is present primarily pyrroles, pyridines, quinolines, indoles, and carbazoles. Amides and oxazoles contain both nitrogen and oxygen. Amines are not found in raw crudes.

Oxygen compounds include naphthenic acids, carboxylic acids, phenols, cresols, and furans.

Trace elements such as Ni and V tend to be incorporated into porphyrins, in which they are chelated by the nitrogen atoms in the porphine ring. Ca- and Fe-containing porphyrins also have been found. Arsenic and mercury are present as alkyl arsenes and alkyl mercury compounds.

Other heteroatom compounds are introduced during production and/or transportation. Iron naphthenates are generated by naphthenic acid corrosion of steel, and organosilicon compounds are added as flow improvers.

The discovery of porphyrins in petroleum added weight to the theory that petroleum came from living organisms. Figure 14 compares the

structure of chlorophyll A with the structure of Ni-containing porphyrin and a V-containing porphyrin. Removal of the side chains, oxygen atoms, and double bonds from the chlorophyll structure, coupled with the replacement of the Mg by either Ni or VO(O), generates the porphyrins.

Continuity Principle

The continuity principle [36] states that the properties of the molecules in homologous series vary monotonically. A homologous series is a group of compounds with the same essential structure, which vary by a single parameter—such as the numbers of –H2C– units in alkyl chains—or the number of condensed rings in polynaphthenes or polyaromatics.

Figure 15 illustrates the continuity principle for atmospheric equivalent boiling point (AEBP). The figure is grossly simplified, but for carbon numbers less than 35, it does correspond with reality. The indicated boiling points are for pure compounds. In mixtures, the boiling points are shifted by molecular interactions. Molecular interactions are greatest for polar compounds, such as those with hetero atoms.

Fig. 14 Comparison of chlorophyll A with a Ni-containing porphyrin (B) and a vanadium-containing porphyrin (C)

Fig. 15 Illustration of the continuity principle. Carbon number and atmospheric equivalent boiling points for different compounds

Boiling points are crucial, because fractional distillation is the primary means by which petro- leum is separated into useful products. Boiling ranges for typical cuts—naphtha, kerosene (including jet fuel), AGO (atmospheric gas oil, including diesel fuel), vacuum gas oils, and residue—appear at the top of the figure. In practice, due to imperfect separation in commercial distillation towers, the fractions overlap.

Curve A is for normal paraffins. Branched isomers with the same carbon number (not shown) boil at lower temperatures. Fully saturated polyring compounds fall on Curve B. Fully unsaturated poly aromatics fall on Curve C. Curve D shows how adding alkyl groups to pyrene makes Curve D parallel to Curve A. Curves E, F, and G represent compounds containing hetero atoms—sulfur, nitrogen, and oxygen.

Phenanthrene (C14H10), a three-ring poly aro- matic compound, is found in the AGO boiling range.   Adding   seven   hydrogen   molecules (14 hydrogen atoms) converts phenanthrene into perhydrophenanthrene (C14H10), shifting it into the kerosene cut.

Crude Assay

Analyses of crude oils are summarized in crude assays. An example crude assay report template is presented in Fig. 16. There is no industry- standard testing grid—each company has its own. Distillation properties determine straight-run yields of key fractions.

Elemental composition and total acid number (TAN) indicates how expensive it will be to process the crude in a refinery. Viscosity, freeze point, and pour point reflect how a fraction will perform in a cold environment. Density, aniline point, and K-factor, along with paraffins, naphthenes, and aromatics, describe molecular composition, which determines       how the fraction        will behave in a refinery.

Cetane number and diesel number are important properties of diesel fuel. The last six properties—heptane asphaltenes, microcarbon residue, Ramsbottom carbon, V, Ni, and Fe—determine to a large extent the cost of upgrading the residue.

References

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