Distillation of Petroleum / Biproducts

In the distillation process, petroleum is separated into different molecular groups :

C1– C4	Gas
C5– C10	Gasoline
C11– C13	Kerosine
C14– C18	Diesel fuel (light gas oil)
C19– C25	Heavy gas oil
C26– C40	Lubricating oil
> C40	Residuum

Natural Gas :

Natural gas usually consists mostly of methane (CH₄) but may contain variable amounts of higher-order paraffins (ethane, propane and butane). “Dry gas” is predominantly methane and ethane, while “Wet gas” contains more than 50% propane and butane. Gas may originate as a byproduct during the generation of oil, from coal or as bacterial gas (swamp gas).

additional components in gas are CO₂, which may be used to make dry ice, and H₂S, which has to be removed because of its toxicity (0.1% is fatal to humans within 30 minutes). At the refinery, H₂S is converted into sulfur by the process known as demercaptenization.

2H₂S + 3O₂ = 2SO₂ + 2H₂O
2H₂S + SO₂ = 3S + 2H₂O

Gas is transported in pipelines, but sometimes also in liquid form on tankers. Liquefied Natural Ga (LNG) is primarily methane, while Liquefied Petroleum Gas (LPG) is largely propane and butane. The latter can be liquified under pressure at room temperature and is cheaper to produce and transport than LNG which has to be kept at low temperatures. Ethylene, propylene and butylene are gaseous olefins that are originally not present on petroleum but are formed in the refining process through cracking of petroleum.

Gasoline :

Gasoline is composed of hydrocarbons ranging from C₅ to C₁₀. The cracking process in the refineries was developed to produce gasoline-sized molecules from higher-order hydrocarbons. Polymerization of smaller compounds also gives gasoline molecules. Combined these two processes can produce up to 70% gasoline from crude oil.

An example of cracking: C₃₀H₆₀ → [CH₄ + C₂H₄ + C₂H₆ + C₃H₆] + [C₇H₈ + C₇H₁₄ + C₈H₁₈]
An example of polymerization: C₃H₆ + C₄H₈ → C₇H₁₄ → C₇H₁₆

• Untreated gasoline shows in combustion engines a phenomenon called “knocking” actually a second, delayed explosion.
• One of the most useful compounds in the gasoline range is Benzene, which serves as a basis for products such as insecticides, weedkillers, dyes, drugs (aspirin), industrial solvents, plastics, nylon fibers, polyurethane foams, rubbers etc.
• Benzene can be synthesized from common naphthene such as hexane and methyl-cyclopentane with the help of a platinum catalyst.

Kerosine :

The next highest group of refining products of petroleum is kerosine with molecules ranging from C₁₁ to C₁₃ . It is the first fraction that shows a significant amount of cyclic hydrocarbons (10-40% aromatics, also naphthenes). Kerosine replaced where oil for use in lamps. Its production can also be increased by cracking during the refining process. The “Flash point” determines below which temperature an oil can be handled safely -without its fumes being spontaneously ignited.

For kerosine, the flash point is considerably higher than for gasoline. This, together with its relatively low freezing point is a main reason for its use as airplane.

Gas oil :

The composition of the oil fraction is complex over it’s total range of C₁₄ to C₂₅. Paraffins are less abundant and more in the form of cyclo-paraffins. Aromatics-mostly polycyclic – as well as non-hydrocarbon compounds increase compared to kerosine.

Light gas oils are used as jet and diesel fules. Diesel engines are compression ignition engines and work differently from combustion engines. Long-chained paraffins that knock badly are very good diesel fuels. Branched and cyclic hydrocarbons can be excellent gasoline but form poor diesel fuels. The quality of diesel fuel is referenced to cetane (normal hexadecane, C₁₆H₃₄).

Lubrication Oils and Waxes :

These compounds range from about C₂₆ to C₄₀, although no fixed limits exist. All types of hydrocarbons can occur here, but compared to the lighter fractions the NSO compounds significantly increase. They give these hydrocarbons their typical Dark color. The amount of wax is mostly determined by straight-chain paraffins.

• The pour point is defined as the temperature at which the oil does stops flowing while cooling down. Straight-chain paraffins increase the pour point, while branched-chain hydrocarbons, cyclic compounds and aromatics lower it. Waxes can be removed with solvents to lower the pour point.
• For a good lubricating oil, the change in viscosity with temperature is important. This is measured with the viscosity index “VI”, which for an oil that does not change its viscosity is 100. This can be the case of paraffinic oils. Aromatics oils may have very low values of VI.

Resins, Asphaltenes and Waxes :

This is the most complex and least understood fraction of petroleum. It is the residue that remains after all lighter compounds have been distilled. The three main groups are separated from each other as follows :

The condensed aromatic structures of asphaltenes ofter contains free radical sites where metallic elements attach, for example vanadium (Va) or nickel (Ni). Both have negative effect on the refining process.

Their principal use is for road construction, furnace oils, binders, filter, insulating material and adhesives. Wax is the paraffin fraction with about C₆₀. Some oils contain over 50% of these compounds. They form important resources but are not heavily produced.