The Chemistry of Petroleum
Introduction:
The smallest unit of a substance, which still retains the characteristics of that substance, is called a molecule. Molecules can only be divided into atoms - which are different elements. For example, all molecules of water are identical and have the characteristics of water. Two atoms of hydrogen and an atom of oxygen (which made up the molecule) on their own have none of the characteristics of water.
Crude oils are mixtures of many different substances, often difficult to separate, from which various petroleum products are derived, such as: gasoline, kerosene propane, fuel oil, lubricating oil, wax, and asphalt. These substances are mainly compounds of only two elements: carbon (C) and hydrogen (H). They are called, therefore: hydrocarbons.
Refining crude oil involves two kinds of processes to produce the products so essential to modern society. First, there are physical processes which simply refine the crude oil (without altering its molecular structure) into useful products such as lubricating oil or fuel oil. Second, there are chemical or other processes which alter the molecular structure and produce a wide range of products, some of them known by the general term petrochemicals.
Hydrocarbons
Hydrocarbons may be gaseous, liquid, or solid at normal temperature and pressure, depending on the number and arrangement of the carbon atoms in their molecules. Those with up to 4 carbon atoms are gaseous; those with 20 or more are solid; those in between are liquid. Crude oils are liquid but may contain gaseous or solid compounds (or both) in solution. The heavier a crude oil (i.e. the more carbon atoms its molecules contain) the closer it is to being a solid and this may be especially noticeable as its temperature cools. Light oils will remain liquid even at very low temperatures.
The simplest hydrocarbon is methane, a gas consisting of one carbon atom and four hydrogen atoms:
A carbon atom has four bonds that can unite with either one or more other carbon atoms (a property almost unique to carbon) or with atoms of other elements. A hydrogen atom has only one bond and can never unite with more than one other atom. The larger hydrocarbon molecules have two or more carbon atoms joined to one another as well as to hydrogen atoms. The carbon atoms may link together in a straight chain, a branched chain, or a ring.
Examples include: Propane (C3H8), a straight chain molecule, shown below.
An example of a branched chain, Isobutane (C4H10), is shown below:
The hydrocarbon rings can become quite complex. As well, more complex molecules
occur when one or more hydrogen atoms are replaced by hydrocarbon groups or by the
condensing or "stacking" of one or more rings. A simple example of this
occurs in naphthalene (C10,H8), shown below:
It is at this point that we are seeing molecules found in crude oil. As the molecular
structure becomes characterized by denser carbon atoms and even further stacking, we
enter the realm of "heavy oil". One of the most complex examples of
"stacking" occurs in asphaltenes, a structure so important in
the challenge of producing and refining heavy oil that we
discuss them separately.
Thus, we see that the number of hydrogen atoms associated with a given skeleton of carbon atoms may vary. When the chain or ring carries the full complement of hydrogen atoms, the hydrocarbon is said to be "saturated".
When less than the full compliment of hydrogen atoms is present in a hydrocarbon chain or ring, the hydrocarbon is said to be "unsaturated".
Another molecular complexity is caused by introducing elements such as sulphur, nitrogen, and oxygen. With these, the number of different molecules based on the carbon skeleton increases tremendously.
Molecular diagrams constructed by Foster Learning Inc. using MoluCad.
