Petroleum Contaminants: Sulfur and other Undesirables
A number of different chemicals can be found in petroleum, but the hydrocarbons are what we are interested in. In fact, other substances in petroleum lead to inefficient burning, pollution, damage to engine components, and other problems. Refining of petroleum is carried out for two reasons. First, it is used to separate hydrocarbons of different sizes to create different fuels and oils. Second, refining is used to remove contaminants. This article looks at a few of the more common contaminants, why they are a problem, and how they are removed.
Sulfur is probably the most common and most well known petroleum contaminant. It is so common, in fact, that oil is rate (sweet or sour) based on its sulfur content. So why do we care so much about sulfur?
Even though early prospectors tasted oil to determine how sweet or sour it was, palatability is not the reason we care about sulfur in oil. A concentration of just 0.5% sulfur will make crude oil “sour,” which means longer refining and more expensive gasoline and other products in the end. Most of this sulfur is found in the form of hydrogen sulfide gas, a poisonous, noxious, foul-smelling gas sometimes called “sewer gas.” Most hydrogen sulfide in petroleum results from the decay of organic matter (the same reason it ends up in sewer gas).
Hydrogen sulfide is actually very flammable, so it could be used as a fuel if it were not for the fact that it is also deadly in relatively low concentrations. Hydrogen sulfide affects the nervous system, respiratory system, and may even have contributed to several mass extinctions in Earth’s past. This deadly gas must be removed from petroleum in order to make it safer for use. The hydrogen sulfide can then be used to produce pure sulfur, a highly valued industrial element used in the production of fertilizer. It is also made into sulfuric acid.
Vanadium is a naturally occurring “heavy metal,” a name that derives from its relatively high atomic mass on the periodic table of elements. Heavy metals are almost universally poisonous, including such familiar names as mercury, lead, cadmium, and arsenic. Vanadium is no exception to this rule and is all vanadium compounds are considered toxic.
Beyond toxicity, vanadium is also an oxidant and is the main component in fuel (particularly diesel fuel) that leads to “high temperature corrosion.” During combustion of hydrocarbon, vanadium in the petroleum reacts with other contaminants like sodium and sulfur to produce compounds known as vandates that increase the corrosion of steel by removing the “passivation” or inert layer that usually shields steel from environmental factors that cause corrosion (rust). In essence, vandates are salts and they increase the rate at which steel engine and exhaust parts rust the way throwing salt on a metal increases the rate at which its rusts. Vanadium also contributes to the corrosion of oil transport pipelines, ships, and tanker trucks.
Iron contaminates oil mainly as a result of corrosion in pipelines, ships, and trucks. The main problem with iron is that it can lead to sludge build-up in pumps, refinery exchangers, and other fuel delivery systems.
Zinc is a unique contaminant because it never occurs as a natural component of oil. The only zinc in petroleum comes from recycling lubricating oils (motor oil and hydraulic oil). The main problem with zinc is that it interferes with the removal of salts from petroleum. Increased salt levels mean increased corrosion of refinery systems, engine parts, etc.