Crude oil, also known as petroleum, is a complex mixture of hydrocarbons. This dark, viscous fluid contains thousands of individual compounds, each made up of different chains of carbon and hydrogen atoms. In its raw state, crude oil is largely unusable for most modern applications, requiring a rigorous refining process. The primary goal is to separate the crude mixture into useful, distinct fractions, each possessing similar properties and boiling points. This separation isolates specific hydrocarbon groups that can be converted into fuels, lubricants, and other valuable products.
Preparing Crude Oil for Initial Separation
Before crude oil can enter the main separation unit, it must be cleaned through desalting and dehydration. Crude oil often contains water, dissolved salts, and suspended solids that cause severe problems in the refinery equipment. Water presence can lead to corrosion inside the distillation tower due to the formation of hydrochloric acid.
The desalting process involves mixing the crude oil with wash water to dissolve and dilute the brine present in the oil emulsion. This mixture is then subjected to a strong electrostatic field within a vessel. The electric field causes the tiny water droplets containing the dissolved salts to coalesce and form larger droplets.
These enlarged, heavier water droplets separate from the oil under gravity, settling at the bottom of the vessel where they are drawn off. Removing these impurities prevents fouling on the heat exchanger and furnace tubes, which reduces the efficiency and lifespan of the refining infrastructure. The resulting desalted and dehydrated crude oil is then ready for heating.
Fractional Distillation: The Main Process
The fundamental method for separating crude oil is atmospheric fractional distillation, which exploits the difference in boiling points among hydrocarbon molecules. The prepared crude oil is first heated in a furnace to an extremely high temperature, often reaching 350 to 400 degrees Celsius, causing most components to vaporize. This hot mixture is then pumped into the base of a tall, cylindrical fractionating column.
The column operates based on a temperature gradient, being hottest at the bottom where the vapor enters and gradually cooler towards the top. As the hot petroleum vapor rises, it cools. Hydrocarbons with higher boiling points condense back into liquid form at the lower temperatures found higher up the column.
The column contains a series of horizontal trays that capture the condensed liquid fractions. Heavier hydrocarbons condense quickly, collecting at the lower, hotter sections. Conversely, the lighter molecules continue to rise as vapor until they reach the cooler upper sections before condensing.
This continuous process allows for the effective separation of the complex mixture based on molecular size. Each tray collects a specific fraction, which is drawn off from the sides of the column at various heights.
The Resulting Hydrocarbon Products and Applications
The separation by fractional distillation yields a variety of distinct products, with the lightest, most volatile fractions collected at the top and the heaviest collected at the bottom. At the very top of the column, the lowest-boiling components are collected as refinery gases. These include methane, ethane, propane, and butane, commonly used as liquefied petroleum gas (LPG) for heating and cooking.
Just below the gases is the gasoline fraction, composed of shorter carbon chains (C5 to C12), which is the primary fuel for internal combustion engines. Naphtha is another light fraction, often used as a solvent or as a feedstock for the petrochemical industry to produce plastics and chemicals. Kerosene, a slightly heavier fraction, is the main component of jet fuel for aircraft.
Further down the column, the middle distillates are collected, primarily consisting of diesel fuel and gas oil. Diesel, which has a boiling point range typically between 200 and 350 degrees Celsius, is the heavy-duty fuel for trucks, trains, and marine vessels.
The heaviest fractions that condense lower in the column include lubricating oils, used to reduce friction in machinery, and heavy fuel oil for ships and industrial boilers. The very bottom collects the non-volatile residue, such as asphalt and bitumen, which is primarily used for paving roads and roofing materials.