Crude Oil CharacteristicsCrude oils consist of mixtures of hydrocarbon molecules plus modest quantities of impurities, such as oxygen, nitrogen, sulfur and trace metals. The chemical composition of each crude oil is different because of the manner in which it was "cooked" during its original subsurface transformation from organic matter to crude oil. This diversity is illustrated in Figure 4, which shows samples of crude oils ranging from a heavy "black oil" on the right to a light "condensate" on the left.

Figure 4: Samples of crude oils from different petroleum reservoirs ranging from the heaviest and most viscous on the right and the lightest, a condensate, on the left.

Crude oil, in its produced state, has limited uses. Its value lies in the many petroleum products that are obtained through refining and supplemental processes. To appreciate the purpose of the various refining processes it is important to learn more about the chemistry of crude oils and how they are transformed into final products.

Crude Oil Chemistry

The number of ways in which hydrogen and carbon can combine to form different hydrocarbons is immense, as are the number and types of hydrocarbon molecules that are found in any given sample of crude oil.

A hydrocarbon molecule's physical state, at a given temperature and pressure, depends on how many carbon atoms it contains (carbon number). Under ambient conditions, that is, standard atmospheric conditions of pressure and temperature, hydrocarbons containing up to four carbon atoms are usually gases (e.g., Methane CH4 or carbon number C1), those with five to nineteen atoms are usually liquids and those with twenty or more are solids. Most hydrocarbon molecules are

found naturally in crude oils; however, many more are "made" during refining or petrochemical processing.

Fortunately, from a refining standpoint, it is not necessary to analyze each of the thousands of hydrocarbon molecules that make up a particular crude oil. Instead, the hydrocarbon molecules can be classified into just four major groups, based on (1) the proportions of hydrogen and carbon atoms in a molecule or (2) the molecular structure—that is, the manner in which the atoms combine with one another. The four groups that form the foundation of all refined petroleum products are:

Paraffin Olefins and Aromatics Napthene or Cycloalkane Other hydrocarbons: Alkenes, Dienes and

Alkynes

Paraffin (Alkane)

Paraffin (also called alkane) molecules consist of carbon atoms linked to each other in a chain, surrounded by hydrogen molecules. The carbon atom attaches to four atoms; hydrogen to one, so their natural structure is given by the general chemical formula CnH2n+2 (n is a whole number, usually from 1 to 20, e.g., CH4, C2H6, C3H8). The carbon atoms in these molecules can be configured either as straight chains (normal) or branched chains (isomers). An isomery is a variation in the arrangement of atoms in two or more molecules that have the same chemical formula. The lighter, straight chain paraffin molecules are found in gases. Examples include methane, ethane, propane and butane (gases containing from one to four carbon atoms), and pentane and hexane (liquids with five to six carbon atoms). Note that a hydrocarbon molecule is often abbreviated by its carbon number, as C3 or iC4.

Figure 5: Shows the structure of the lighter paraffin molecules from methane, the lightest of the hydrocarbon molecules, to isobutane. Methane is sold as natural gas and burns with a characteristic blue flame. Ethane is sold as a gas or converted to ethylene as the first step in making plastics. Propane and butane are the LPGs, which are used directly as fuels, as blending fractions to improve the characteristics of gasoline or in the manufacture of petrochemicals.

The four lightest molecules are shown in Figure 5. Methane (C1), the lightest molecule with only one carbon atom and four hydrogen atoms, is the natural gas that is delivered to our homes by distribution companies. Its molecular weight is 16 (1 carbon with a molecular weight of 12 plus 4 hydrogen atoms with a molecular weight of 1 = 16). Ethane (C2), the second lightest molecule, has a molecular weight of 30, twice the weight of methane, and is usually converted in a special high-temperature "steam cracking" process to make ethylene (C2H2), the first step in making plastics. The next two paraffin molecules, Propane (C3) and Butane, are referred to as the Liquid Petroleum Gases, or LPGs. They are sold individually or as a mixture in metal containers, under pressure, to maintain their liquid state. We use these as fuels for cooking (outdoor barbecue), home heating or vehicle fuel. They are also used in making petrochemicals. Note how Normal Butane (C4) is a straight chain molecule and Isobutane (iC4) is an isomer; however, both have the chemical formula C4H10. The difference in structure causes them to have different physical properties.

There are many more, longer chain paraffin molecules that may be part of the crude oil's molecular mixture. They exist in sequence from the lighter liquids range of normal and isopentane (C5) to the familiar octane molecule (C8) and on to paraffin wax in the range of (C20 - C40) and to even heavier molecules. We will discuss some of these later in the module.

All paraffin is fully saturated hydrocarbons; this means that every carbon atom is bonded to four other atoms, the maximum number possible when there are no double bonds. It also means that they are stable and not likely to react with other molecules.

Olefins and Aromatics

If you place ethane molecules in the presence of steam for one second at a very high temperature (1600oF or 871oC) the molecule will "crack" ethylene", which, as shown in Figure 6, has a double bond on the carbon atoms and only four hydrogen atoms. This new molecule, an olefin, must be manufactured - they are not found naturally in crude oils. Other olefins can be made through steam cracking during refining and petrochemical processes: for example, propylene and butylene.

Figure 6: Shows three members of the olefin family. Note the difference from the paraffin molecule – the double bond and the loss of hydrogen atoms.

Because of the double bond on the carbon atom, olefin molecules are unstable, which means that they can be chemically reacted with other compounds to make a new compound and, in the process, eliminate the double bond. Thus, for example, ethylene molecules can be readily bonded together to make polyethylene, the building block of plastic products.

Aromatics are ring-type molecules, like the Benzene and Toluene molecules shown in Figure 7. They may contain more than one ring. Their carbon atoms are bonded to fewer than four other atoms; in other words, they have carbon atoms that are deficient in hydrogen, and, to make up for this, they have a double bond. For this reason, they are unstable and react readily with other molecules. The most complex aromatics, polynuclears (three or more fused aromatic rings) are found in heavier fractions of crude oil.

Figure 7: Shows two aromatic molecules, one with one ring, the other with a CH3 radical, rather than a hydrogen atom attached to one of the carbon atoms. Note how they are cyclic in nature, a single bond next to a double bond and have double bonds where the hydrogen molecules are missing. Remember carbon normally attaches to four atoms. Because the aromatic molecules are not stable they react with other molecules.

Naphthene

Naphthene or Cycloalkene are saturated hydrocarbon groupings with the general formula CnH2n, arranged in the form of closed rings (cyclic). They are found in all fractions of crude oil except the very lightest. Single-ring naphthene (monocycloparaffin) with five and six carbon atoms predominates, with two-ring naphthenes (dicycloparaffin) found in heavier ends of naphtha. Figure 8 shows the chemical structure of two naphthene (Cyclohexane and Methyl Cyclopentane) found in typical crude oils.

Figure 8: Shows two typical napthene molecules, each with the same chemical formula (C6H12) but different molecular structures. The napthenes are stable molecules.

We see, then, that crude oils are complex mixtures of hydrocarbons and modest amounts of non-hydrocarbon molecules. They range in size from the smallest molecule, methane, CH4, to the heavy asphaltene molecule, C57H32, and even heavier, C85H60. They all have market value but not as a mixture in crude oil. They must be separated and upgraded into marketable products.

Heating Value of Hydrocarbons

Because most hydrocarbons are used as heating or transportation fuels, the thermal energy content of the various hydrocarbon molecules is an important characteristic of its value. Note in Figure 5how the heating value (BTU = British Thermal Unit, the thermal energy generated by a unit volume of the hydrocarbon molecule during combustion), increases as the molecule becomes longer and heavier. For example, a cubic foot of propane gas used in your outdoor grill gives off about 2.5 times the thermal energy of a cubic foot of methane on your kitchen stove. Likewise in Table 1 you could note that the heating value of Decane is about 2.5 times of normal butane.

Hydrocarbon Heating Values

Hydrocarbon Formula Heating Value @ 60 °F

    Btu/gal

Hydrocarbon Heating Values

nButane nC4H10 3262

nPentane nC5H12 4009

Octane C8H18 6249

Decane C10H22 7743

Table 1: Shows the heating values of a number of hydrocarbon molecules.

Boiling Points of Hydrocarbons

Refining processes rely strongly on the fact that hydrocarbon molecules have different boiling points (the temperature, at atmospheric pressure, at which a molecule, when heated, vaporizes and becomes a gas or, on cooling, condenses and becomes a liquid). Note that the boiling points of all five molecules in Figure 5 are different, that they increase as the molecule becomes heavier and are all below 60ºF (15ºC). This means that these five molecules are all gases at standard temperature and pressure.

Hydrocarbon Boiling Points

Hydrocarbon Formula Boiling Point

    (ºF) (ºC)

nButane nC4H10 31 -0.5

nPentane nC5H12 97 36

Octane C8H18 257 125

Decane C10H22 345 174

Dodecane C12H26 421 216

Eicosane C20H42 649 343

Triacontane C30H62 842 450

Table 2: Shows the boiling point of a number of hydrocarbon molecules.

Table 2 lists the boiling points of a selection of hydrocarbons at atmospheric pressure. They have a very wide range of values, from a low for nButane to a high for Triacontane. This illustrates an important characteristic of crude oils. We remember that water, when placed in a teapot and heated, will warm up until it reaches its boiling point of 212oF (100oC), and then boil at that temperatureuntil all of the water has been converted to vapor (see Figure 9).

Figure 9: As water is heated it reaches its boiling point and then remains at that temperature until all of the water has been converted to vapor.

Water, however, is a single component mixture of only water molecules, H2O, which all have the same boiling point. Crude oils, on the other hand, are mixtures of many different molecules, each with different boiling points and so, as a crude oil is heated, there is a progression of boiling points - the lighter molecules vaporize at the lower temperatures and, as the temperature rises, progressively heavier molecules boil and are converted to vapor. So, as we see in Figure 10, the volume percent of the crude oil that is converted to vapor increases as the temperature rises. This is called the distillation curve for that crude oil.

Figure 10: The distillation curve for a typical crude oil is shown here. It is really a plot of boiling point temperatures of the various molecules in the crude versus the cumulative volume that has been converted to a vapor at that temperature.

The refining industry has found it convenient to divide distillation curves into mixtures of petroleum molecules that boil between specified temperature ranges ("cut points"). When the molecules in a given temperature range are then condensed to the liquid state they are referred to as fractions or cuts. The generally accepted temperature ranges or "cut points" are shown in Table 3 with the name given to each cut. They are also shown in Figure 10.

Fraction Temperatures

Butanes and lighter less than 90 °F

Naphtha 90 - 200 °F

Gasoline 200 - 350 °F

Kerosene 350 - 450 °F

Distillate Gasoil 450 - 650 °F

Fraction Temperatures

Vacuum Gasoil 650 - 1000 °F

Residue 1000°F and higher

Table 3: This table shows the temperature ranges, or cut points, for the various petroleum fractions of a crude oil separated during distillation.

Because crude oils differ widely in composition, their distillation curves will have substantially different profiles. This is shown in Figure 11. Note that the lighter crudes have a higher percentage of lighter molecules and so tend to the right in the chart, while the heavier crudes tend to the left, which means that they contain a higher volume percent of heavier molecules. We will discuss these fractions in greater detail as we proceed through this module, during which you will learn that distillation and the separation of crude molecules into different fractions are important aspects of the refining process.

Figure 11: The distillation curves of crude oils differ significantly because of the differences in their hydrocarbon composition.

Volume Changes During Refining

As the heavier fractions of a crude oil are heated above 900°F (see distillation curves above) an interesting phenomenon takes place. As the temperature increases from 900 to 1100°F, the shape of the distillation curve changes and more than 100% of the original volume is recovered. This is caused by the larger molecules cracking into two or more molecules at the higher temperatures. For example, Cetane (C16H34) may be cracked at these temperatures into the three molecules, Octane (C8H18), Hexane (C6H12) and Ethylene (C2H4). The smaller molecules, with lower boiling points, immediately vaporize, changing the characteristic of the distillation curve. Moreover, because the smaller molecules weigh less per unit volume than Cetane, a fixed weight of Cetane will be converted into a fixed weight of these three molecule but the three molecules will expand to fill a greater volume. Thus the overall weight of the molecules stays the same but the volume expands. The cracking of 1 cubic meter of the Cetane molecule will be transformed into 1.38 cubic meters of the three cracked molecules! Nothing magic, it's just molecular structure!

Other refining processes cause smaller molecules to join together and form larger molecules. This reverse process causes the volume of the new molecules to shrink because the larger molecules weigh more per unit volume. We will return to this change in volume phenomenon a little later.

Crude Quality: API Gravity and Sulfur Content

Crude oils are classified and priced according to their density and sulfur content. Both of these properties are measured in a crude oil sample and are included in the crude assay that is provided to the potential buyer by the producer. In order to value a crude oil accurately, the complete crude oil assay is required.

API Gravity

Crude oils are commonly classified as light, medium, heavy or extra heavy, referring to their specific gravities as measured on the American Petroleum Institute (API) Scale. The API gravity is measured in degrees and is calculated using the formula:

API Gravity, ºAPI = (141.5 / specific gravity) – 131.5

Specific Gravity = density of crude / density of fresh water

Consider, for example, a crude oil that has a density of 0.88 g/cm3, compared with fresh water having a density of 1.0 g/cm3. Its specific gravity and API gravity are:

Specific Gravity = 0.88 / 1.0 = 0.88 API Gravity = (141.5 / 0.88) – 131.5 = 29

ºAPI

We note that the two scales move in opposite directions – the lighter the crude, the higher its API number. The petroleum industry defines light crude oil as having an API gravity greater than 31.1°,medium oil as having API gravity between 31.1° and 22.3°, heavy oil as having API

gravity between 22.3° and 10°, and extra heavy oil as having an API gravity of less than 10°. Light crudes are easier for a refiner to process than heavy crudes.

Figure 12 shows the correlation between crude specific gravity and ºAPI. It also shows the API gravity of several different crude oils, including the heavy Lagunillas crude of Venezuela (21.5º), the medium grade Prudhoe Bay Alaska (26º), the Arabian Light crude produced from the giant Ghawar Field, Saudi Arabia (34º), the offshore UK offshore Ninian Field crude (35º), which makes up one of the Brent crudes, the very light condensate produced from the Arun Field (54.7º), which also supplied a major source of gas for an Indonesian LNG project, and water (10º).

Figure 12: The relationship between specific gravity and °API are shown here for several prominent crude oils. So-called "lighter" crudes have higher API gravities and lower specific gravities.

Sulfur Content

Sulfur content—the percentage of sulfur compounds found in crude oil—is of major importance to a purchaser of crude oil.

Crude oils with high sulfur content (greater than 0.7% sulfur by weight) are called sour, while those with low sulfur content (0.7% by weight or less)

are called sweet. Each crude oil grade will have its own sulfur concentration, but typically, the proportion, stability and complexity of sulfur compounds increase as crudes become heavier. High sulfur crudes require additional processing to meet regulatory specifications for refined petroleum products. Figure 13 is a plot of API gravity and sulfur content for a variety of crude oils, from the heavy sour crudes in the upper left to the light sweet crudes in the lower right.

Figure 13: This plot of crude oil sulfur content versus API gravity shows, in general, that the heavier the crude, the more sour it is. The red line near the center of the chart shows the average worldwide crude quality trend line. It indicates that the average crude quality has been moving toward heavier, sourer crudes since 1980. This has strong implications on the need for more sophisticated refineries to produce environmentally acceptable products. Source: Oil & Gas Journal Assay Survey.

Figure 14: Shows that about 66% of the crude oil produced today islight/medium sour, 22% is light sweet and the balance is heavy sour. The most quoted benchmark crudes are light sweet crudes, as we discuss below. However, the majority of crude marketed today is light/medium sour.

Marker Crudes and Pricing Benchmarks

Crude oil benchmarks were created in the mid-1980s as a way of setting market prices for other crudes. The three most widely used benchmarks (or "marker crudes") are West Texas Intermediate (WTI) from the US, Brent Blend from the UK North Sea and Dubai, or Fateh, from the United Arab Emirates. These crudes are shown relative to other crudes in Figure 13.

All other crude oils are valued based on their ºAPI gravity, sulfur content, location and other market factors relative to these benchmark crude oils. For example, Dubai is a light sour crude, but distant from consuming centers. For that reason, it tends to sell at a lower price than Brent and WTI. Table 4 provides the name, location, classification, ºAPI gravity and sulfur content of these three majorMarker Crudes.

Name Location Classification°API

Gravity

West Texas Intermediate Cushing, Oklahoma, USA light, sweet 39.6

Brent Blend North Sea, UK light, sweet 38.1

Name Location Classification°API

Gravity

Dubai/Fateh Dubai, UAE light, sour 30.4

Table 4: Shows the key characteristics of the three major marker crudes. These crudes set the benchmark prices against which other crudes trade.

West Texas Intermediate has traditionally been the highest valued crude oil, typically trading at a $1.50 to $2.50 per barrel premium over Brent Crude because of its location and its slightly higher quality that can be used by a big range of refineries. Thus, it has been the global benchmark in crude oil demand. However, in recent years, Brent Crude prices surpassed WTI prices for what analysts believe is declining production in the North Sea and an over-supply of WTI at its trading hub. Figure 15 shows the historical spot prices of WTI and Brent crudes.

Figure 15: Market price history for WTI and Brent Marker Crudes. Note that Brent is trading at a higher price than WTI since last year.

Interactive Chart: Mouse over the graphic to see individual values. Turn lines On or Off by clicking on the labels below the graphic. Click and drag to zoom in the chart.

Crude Quality: Yield of Petroleum Fractions

The yields of petroleum fractions during distillation for a range of crude grades from light sweet to heavy sour are shown in Figure 16 below. Light sweet crudes, like WTI and Brent, for example, have a relatively high API gravity, about 34-39°API, low sulfur content and are more valuable in the international oil market. They yield about one-third gasoline, one-third distillates and one-third heavy products. These crudes satisfy about 35% of world demand.

Figure 16: Shows the characteristics, market demand and yields during distillation of three different crude types. The charts on the right show the product market demand in the US and Europe. Note that the product yields of the three crudes do not satisfy market demand in these two major markets.

Medium sour crudes, like Mars in the Gulf Coast and Arab Light and Arab Medium in the Middle East have lower API gravities, between 24-34° API, and are sour with greater than 0.7% sulfur content. Their yield during distillation is about one-half heavy products and the balance is divided equally between gasoline and distillates as shown here. They satisfy about 50% of world demand and cost less than the light sweet crudes.

Heavy sour crudes, such as Maya from Mexico, Cerro Negro from Venezuela and Cold Lake from Canada, have gravities below 24° API and sulfur content greater than 0.7%. As you can appreciate, they yield a much lower percent of lighter products, 15% gasoline and 21% distillates, and much higher, approximately 63%, heavier products. They satisfy the remaining 15% of world demand and are the least costly crudes.

Note also that the heavier the crude the greater the quantity of heavy ends, reaching 63% for the heavy sour crude. Also note that the medium sour crudes satisfy about 50% of demand and are less expensive than the light sweet crudes, which satisfy 35% of the market today.

On the right side of Figure 16 are plots of the product market demand in two major market regions: the US and Europe. Notice that none of these crudes satisfy the relative market demands for products in these two major market areas. This means that refining processes are needed to rearrange the existing hydrocarbon molecules to make lighter products, especially in the US, and to enhance the quality of the final product to

meet performance and environmental standards. We will turn to these important refining processes in the next section.