Crude Oil Distillation

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Crude oil is separated through fractional distillation. This is the process of separating the oil mixture into its separate components by use of their different boiling points.

This is first done under atmospheric pressure. Under these conditions, this form of distillation is known as atmospheric distillation. However, after atmospheric distillation, there is a mixture of compounds left over, called atmospheric residue, which has to be broken down as well.

To do so, the next step is vacuum distillation. The fractional distillation process is repeated, this time under vacuum. Because the residue from atmospheric distillation has a very high boiling point at atmospheric pressure, putting it under vacuum means it can be broken down at a lower temperature. This avoids the compounds degrading, also known as cracking.

Crude oil in its raw form, as it is drawn up from underground reservoirs, can be used as a fuel, but is much more useful and valuable when it is broken down into its fractions. Crude oil distillation is this important step.

Distillation separates the different compounds so that they can be used individually. These can be fuels, like kerosene, diesel and petroleum, or raw materials for other chemical processes, like naphtha.

In order to increase its efficiency and prevent unwanted byproducts, distillation takes place in two steps: firstly under atmospheric pressure, then under vacuum.

Crude oil is a mixture of a variety of different hydrocarbons, all of which have different boiling points. We must therefore talk of a boiling range, rather than a specific boiling point.

The range is counted from the lowest boiling point, when the first drop of product is obtained, and the final boiling point, when the compounds with the highest boiling points evaporate. This can mean a range as large as between 20°C and 500°C.

At atmospheric pressure, the first to vaporize is liquid petroleum gas at 25°C. The last, at more than 350°C, is bitumen. This is usually drawn off with other atmospheric residue, which have even higher boiling points and require vacuum distillation to separate.

All other common fuel and lubricating oils have boiling points that fall between these two extremes.

As its name suggests, vacuum distillation is distillation that takes place below atmospheric pressure, under vacuum. This allows heavier oils with a high boiling point to vaporize at lower temperatures, while ensuring that lighter oils do not degrade, or crack.

Crude oil is predominantly a mixture of different hydrocarbons – chemicals made up of hydrogen and carbon molecules.

Generally, this accounts for approximately 98% of the crude oil’s overall weight. The remaining 2% comprises other elements, including nitrogen, oxygen, sulfur and various metals.

To separate these hydrocarbons and extract useful oils, the crude oil undergoes fractional distillation, with part of this process taking place under vacuum.

At the end of the petroleum refining process, there are many heavy fuel oils left over. While these can be used, they are not necessarily the most profitable product. The petroleum industry therefore makes use of a process known as cracking.

The heaviest oils have long chains of hydrocarbon molecules, which cracking breaks into shorter ones. This creates lighter fractions – the more profitable hydrocarbons, such as jet fuel, or car fuel like diesel or gasoline.

Cracking can also occur as a side effect in the distillation tower. If the temperature is too high, the hydrocarbons with the lowest boiling points will begin to break down. However, unlike in the refining process, this is uncontrolled and may not produce the hydrocarbons that are actually desired, or useful. To prevent this, vacuum distillation is used. It lowers the boiling points of the fractions, meaning that they can separate at lower temperatures.

To reduce the boiling point of the residual crude oil mixture, it is put under vacuum. The pressure is in the rough vacuum range, between 10 and 50 hPa (mbar). As a result, the temperature in the distillation tower only needs to reach a maximum of around 370°C.

Crude oil’s main use is as a raw material from which various types of fuel can be extracted, such as heating oils, petrol (gasoline), and jet fuel.

However, crude oil is also the raw material behind many everyday products we find around us. From window frames to food packaging, smart phones to ballpoint pens, crude oil is the base ingredient of all plastics.

However, in order to create useable oils from this raw material, crude oil must be separated through distillation, which takes place partially under vacuum.

The temperature of crude oil distillation is directly related to the boiling points of the different fractions. These boiling points range from under 25°C to 500°C.

However, hydrocarbons can degrade or crack at temperatures above 370°C. As a result, crude oil is never heated above this temperature. Fractional distillation at atmospheric pressure is therefore carried out at temperatures of around 350°C.

In order to separate the last compounds with the highest boiling points, the residual mixture is distilled under vacuum. This lowers the boiling points to below 370°C, protecting the fractions from degrading.

The name petroleum has two meanings. It can refer to both unrefined, unprocessed crude oil and to the petroleum products that it can be turned into during refining. The name is often used interchangeably to mean both.