Wired Chemist

Phyllis A. Leber
Department of Chemistry
Franklin and Marshall College
Lancaster, PA 17604

Introduction

Petroleum is a mixture of hydrocarbons and their derivatives that occur naturally in the earth. Although liquid petroleum, more commonly known as crude oil, contains light or volatile hydrocarbons such as octane (C8H18), it also consists of gases, waxes (solid hydrocarbons), and bituminous materials such as asphalt and tar.

Petroleum, which is distributed liberally through the earth's crust, has been utilized as a heating and construction material since antiquity. For example, an asphalt ring dating from the Sumerians in 3500 B.C. has been excavated at the site of the prehistoric city of Ur in southern Babylonia.1

For the next 5450 years the earth's population exploited little of its oil reserves. However, in the last 50 years major world oil dependency has been prompted by the rapid rise of mass transportation and the emerging petrochemical industry after World War II. A flourishing world oil trade has evolved as a consequence of the uneven distribution of the world's oil reserves. While oil is pumped through extensive pipelines on land, several thousand oil tankers traverse the world's seas as components of an extensive petroleum transportation system. Although oil consumption relative to other energy sources has decreased modestly during the last decade, Figure 1 clearly shows that oil is still a dominant energy source.

Origin of Petroleum

Because it is derived from decaying plant and animal debris, primarily phyto- and zooplankton,a petroleum is referred to as a fossil fuel. While the water-insoluble organic components of these deposits initially contained oxygen, oxygen was largely removed by two processes. The elimination of water, or dehydration, was accomplished under conditions of high pressure from accumulating sediment, while decarboxylation, the evolution of carbon dioxide occured due to the action of anaerobic bacterial decomposition. Definitive evidence favoring theories that connect petroleum to organic rather than inorganic sources stems from the observation of optical activity among certain petroleum constituents. Plausible source materials for the porphyrinb constituents of crude oil are chlorophyllc and heme.d Additionally, fatty acidse are prime candidates as petroleum source materials because they can be readily converted to hydrocarbons by decarboxylation (equation (1)).

RCO₂H → RH + CO₂(g) (1)
fatty acid hydrocarbon
for example:
CH₃(CH₂)₁₅CH₂CO₂H → CH₃(CH₂)₁₅CH₃ + CO₂(g)
stearic acid heptadecane

Although oil originates in a sedimentary marine source bed, it ultimately migrates through faults and fractures created by tectonicf plate movement into reservoir rock. Reservoir rocks are usually sedimentary rocks such as sandstone or limestone whereas typical cap rocks, which prevent the escape of the volatile components of oil, are clays and shales that are far less permeable. Stratificationg into discrete gas (i.e., methane), oil, and water layers often occurs in the reservoir rock.

Composition of Petroleum

The proportion of oily constituents has been used as the key criterion in differentiating petroleum and asphalt. Petroleum is usually characterized as containing >50% oily constituents.

A complex mixture of substances, petroleum consists predominantly of hydrocarbons with metals and compounds containing nitrogen, sulfur, or oxygen present in minor quantities (Table 1). The hydrocarbon components of petroleum fall into three classes: paraffins or saturated hydrocarbons, either branched or unbranched (C_nH_2n+2), cycloparaffins or naphthenes (C_nH_2n where only one ring is present), and aromatics such as benzene (C₆H₆). The non-hydrocarbon components consist of either metals or sulfur-, oxygen-, or nitrogen-containing compounds.

Interconversion among the various hydrocarbon constituents of petroleum occurs via hydrogen addition or loss . It is probable that these chemical transformations occur during the formation and development of oil reserves, possibly due to the catalytic effect of cavities in porous clay.

Table 1. Hydrocarbon Constituents of Petroleum

Hydrocarbons	Examples
Paraffin
Naphthenes
Aromatics
Non-Hydrocarbons	Examples
Sulfur Compounds
Oxygen Compounds
Nitrogen Compounds
Metals	Fe, Ni, V, Cu

 

Properties of Petroleum

Due to their hydrophobic or nonpolar nature, hydrocarbons are not soluble in water. Because they are less dense than water, hydrocarbons float. Crude oil typically has a density of 0.85 g/mL.

Volatility is one of the fundamental characteristics of liquid fuels. As such, it forms the basis for the characterization of liquid petroleum fuels such as liquefied petroleum, gasoline, aviation fuel, naphthas, kerosene, gas oils, diesel fuels, and fuel oils.

Petroleum Refining

Separation of commercially important components of petroleum is achieved using a process known as fractional distillation. In fractional distillation, a vertical fractionating column with a large surface area is utilized to improve distillation efficiency. Commercial fractionating columns (Figure 2) consist of many horizontal plates that allow for repetitive vaporization-condensation cycles of the distillate so that the vapor reaching the top of the column is highly enriched in the more volatile components of the distilland. This fractionation is achieved by the establishment of a temperature gradient within the fractionating column; thus, the temperature at the top of the column is lower than is the temperature at the bottom of the column. Ideally, the temperature at the top of the column will correspond to the boiling point of the most volatile component of the original mixture.

Because hydrocarbons are essentially nonpolar in nature, separation of hydrocarbons occurs as a function of increasing molecular weight, which correlates with increasing boiling point (Table 2). For the series of petroleum constituents listed in Table 2, the order of emergence from a fractionating column parallels the boiling point: pentane, benzene, cyclohexane, octane, decalin, phenanthrene. Practically speaking, it would be difficult to separate completely benzene and cyclohexane due to the similarity in their boiling points. Fractional distillation of petroleum affords numerous fractions as a crude function of carbon number and boiling point (Table 3).

 

Table 2. Representative Examples of Petroleum Constituents

Name	Formula	MW (g mol-1)	BP (°C)
Butane	C4H10	58.13	-0.5
Pentane	C5H12	72.15	36
Benzene	C6H6	78.12	80
Cyclohexane	C6H12	84.16	81
Octane	C8H18	114.23	126
Decalin	C10H18	138.25	196
Phenanthrene	C14H10	178.24	340

 

Table 3. Approximate Composition and Boiling Points of Various Petroleum Hydrocarbon (HC) Fractions
Source: Reference 2, p 114.

Name of Fraction	Representative HCS	Approximate BP (°C)
Natural gas	CH4	-161
Liquefied gas (LP gas)	C3H8, C4H10	-44 to +1
Petroleum ether	C5H12, C6H14	30-60
Aviation gasoline	C5 to C9	32-150
Auto gasoline	C5 to C12	32-210
Naphtha	C7 to C12	100-200
Kerosene	C10 to C16	177-290
Fuel oil	C12 to C18	205-316
Lubricating oils	C15 to C24	250-400

Octane Rating

Gasoline is one of the commercially important fractions that result from petroleum refining. In the internal combustion engine of an automobile, energy is produced by the ignition of a compressed mixture of air and gasoline. The combustion of the gasoline creates gases, carbon dioxide and water vapor (equation (2)) , the pressure from which creates mechanical work by forcing a piston down a cylinder. Unbranched hydrocarbons such as octane have a great tendency to ignite spontaneously during this process. This phenomenon of pre-ignition is known as "knocking," which reduces power efficiency and increases engine wear.

C₈H₁₈ + (25/2) O₂ → 8 CO₂(g) + 9 H₂O(g) (2)
octane

Octane rating is a numerical scale that reflects the relative tendency of a gasoline component to cause engine knocking. On this scale heptane is assigned a rating of zero; 2,2,4-trimethylpentane (isooctane), 100. Because octane results in even more knocking that heptane, its rating is -19. Gasoline with an octane rating of 87 is equivalent in "knocking" potential to a mixture of 87% isooctane and 13% heptane.

Aromatics generally have higher octane ratings: benzene, 105; toluene, 120. However, concerns about the toxicity of the more volatile aromatic components of petroleum (BETX) - benzene, ethylbenzene, toluene (methylbenzene), and xylene (dimethylbenzene, which can exist as the ortho, meta, or para isomers) - has resulted in a modest relaxation of desirable octane rating. Octane ratings of 86-95 are frequently encountered at the gasoline pump. Octane ratings less than 100 ensure a minimal contribution from aromatic hydrocarbons.

Prior to 1970, tetraethyllead was routinely added to gasoline to increase its octane rating. Addition of 2.5 to 4.0 g of lead (3.9-6.2 g of tetraethyllead) per gallon of gasoline can raise 87 octane gasoline to 93 octane. However, the use of lead additives represented a health risk due to the presence of finely-divided particles of PbClBr in automobile exhaust. In 1968, it was estimated that more than 98% of lead emitted in the atmosphere originated from gasoline combustion.2 To compensate for the reduction in octane rating that would result by eliminating lead additives, automobile manufacturers lowered the compression ratios of automobile engines to permit the use of lower octane fuel.

The conversion to lead-free gasoline has also been dictated by the use of catalytic converters (which would be poisonedi by lead) on automobiles since 1975, mandated by the Clean Air Act of 1970, to reduce nitrogen oxide pollution. Nitrogen dioxide is produced as a byproduct of combustion. Photodissociation of NO₂ yields two reactive species: NO can recombine with O to reform NO₂ or O can combine with O₂ to form ozone (O₃). The primary function of the catalytic converter is to reduce nitrogen oxides in the form of NO to nitrogen (equation (3)). Catalytic converters have resulted in a 76% reduction in the emission of nitrogen oxides.3

2 NO + 2CO → N₂(g) + 2 CO₂(g) (3)

Methods of Petroleum Refining

Due to the importance of enhancing the gasoline fraction in petroleum refining, the chemical processing is designed to increase the composition of the C₅-C₁₀ hydrocarbons (Table 3) of higher octane number, which are typically branched paraffinic, naphthenic, or aromatic compounds. The more common chemical processes in petroleum refining include six categories: cracking, polymerization, alkylation, isomerization, catalytic reforming, and hydrocracking.

Cracking

Heating petroleum at high temperatures (>350°C) favors the breakdown of higher molecular weight components into smaller (lower molecular weight) fragments. For example, thermal cracking of dodecane (C₁₂H₂₆) yields hexane and 1-hexene (equation (4)), both of which are viable gasoline components. Although olefins (alkenes) such as 1-hexene are rarely found in unrefined petroleum, they are abundant in processed petroleum due to the cracking phenomenon.

heat + CH₃(CH₂)₁₀CH₃ → CH₃(CH₂)₄CH₃ + H₂C=CH(CH₂)₃CH₃ (4)
dodecane hexane 1-hexene

Catalytic cracking is essentially thermal decomposition of hydrocarbons in the presence of catalysts such as a crystalline aluminosilicates (zeolites) or molecular sieves. In petroleum refining, catalytic cracking has largely superseded thermal cracking because it produces gasoline richer in branched paraffins, cycloparaffins, and aromatics.

Polymerization

Petroleum constituents containing fewer than five carbon atoms can be converted to higher molecular weight hydrocarbons via acid-catalyzed polymerization. A true polymerization would involve the addition of a monomer, a low-molecular-weight unit, to itself in a repetitious fashion to yield a high molecular weight compound. The reaction here is not a true polymerization process in that it is terminated at either the dimer or the trimer. For example, methylpropene (isobutene), a monomer, can be dimerized to form 2,4,4-trimethyl-1-pentene (equation (5)), an eight-carbon hydrocarbon that would enhance the octane rating of gasoline due to its extensive branching.

(5)

Alkylation

Acid catalysts such as sulfuric acid or aluminum trichloride promote reaction of either a highly-branched paraffin such as isobutane or an aromatic compound with an olefin such as ethylene or propene. In equation (6), we observe that benzene can react with propene to yield isopropylbenzene (cumene).

(6)

Isomerization

The value of isomerization in petroleum refining is twofold: unbranched paraffins can be converted to isoparaffins, thus increasing the octane rating of the gasoline fraction; isoparaffins such as isobutane thus produced can also be alkylated to liquid hydrocarbons in the gasoline range.

Catalytic Reforming

In the process of catalytic reforming, paraffins can first be transformed into naphthenes and then to aromatic compounds. Equation (7) illustrates the catalyzed reformation of hexane to cyclohexane (equation (7a)) and then to benzene (equation (7b)). A typical reforming catalyst is molybdenum oxide.

Hydrocracking

The process of hydrocracking couples catalytic cracking with hydrogenation. The reactions are catalyzed by dual-function catalysts: zeolite catalysts for the cracking function and platinum, nickel, or tungsten oxide for the hydrogenation function. The hydrogenation reactions occur because hydrogen is generated as a by-product in the course of catalytic reforming. In the example cited in equation (7), a total of four equivalents of molecular hydrogen is produced. Thus, the olefins produced in catalytic cracking are reduced to corresponding paraffins (saturated hydrocarbons). Aromatics are also reduced under the conditions employed for hydrocracking. Hydrocracking of naphthalene, for example, yields benzene via the intermediacy of various alkylbenzenes (butylbenzene, ethylbenzene, toluene).