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 inFigure 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 manypetroleum productsthat 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 ChemistryThe 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 usuallygases(e.g., Methane CH4or carbon number C1), those with five to nineteen atoms are usuallyliquidsand those with twenty or more aresolids. 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 justfour major groups, based on (1) the proportions of hydrogen and carbon atoms in a molecule or (2) the molecular structurethat 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 AlkynesParaffin (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 formulaCnH2n+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). Anisomeryis 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 C3or 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 inFigure 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 theLiquid 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 howNormal Butane (C4)is a straight chain molecule andIsobutane (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 isfully saturatedhydrocarbons; 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 AromaticsIf 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 inFigure 6, has a double bond on the carbon atoms and only four hydrogen atoms. This new molecule, anolefin, 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.Aromaticsarering-typemolecules, like the Benzene and Toluene molecules shown inFigure 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.NaphtheneNaphtheneorCycloalkeneare 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 8shows 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 HydrocarbonsBecause 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 inFigure 5how theheating 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 inTable 1you could note that the heating value of Decane is about 2.5 times of normal butane.Hydrocarbon Heating Values
HydrocarbonFormulaHeating Value @ 60 F
Table 1:Shows the heating values of a number of hydrocarbon molecules.Boiling Points of HydrocarbonsRefining processes rely strongly on the fact that hydrocarbon molecules have differentboiling 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 inFigure 5are different, that they increase as the molecule becomes heavier and are all below 60F (15C). This means that these five molecules are all gases at standard temperature and pressure.Hydrocarbon Boili