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Crude Oil Distillation

Vacuum distillation ensures that the different types of oil contained in the crude oil mixture can be separated at a lower temperature.


The role of vacuum in crude oil distillation

When it is extracted from the ground, crude oil is a mixture of many different liquid hydrocarbons.
Under vacuum, the boiling point of liquids can be lowered.
In order to create useful products like diesel fuel, heating oil or naphtha, it must first be broken down into its components through a process called distillation.

However, to ensure that there are no unwanted byproducts and no degrading, the oil must be kept below a certain temperature.

Under vacuum, the boiling point of liquids can be lowered. Vacuum distillation therefore ensures that the different types of oil contained in the crude oil mixture can be separated at a lower temperature.

Busch vacuum technology in crude oil distillation

Our product range includes a large number of different vacuum solutions for crude oil distillation processes. We supply individual pumps and complete vacuum systems.

DOLPHIN liquid ring vacuum pumps are used for the vacuum distillation of crude oil. Their robustness and suitability for hazardous processes make them the perfect solution. They also meet the API (American Petroleum Institute) standards 681 for liquid ring vacuum pumps and 682 for mechanical seals. ATEX-certified versions are available.

Use our product finder to find the best solution for your process.

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Vacuum applications in crude oil distillation

Crude oil distillation is the process undertaken in oil refineries to separate the different hydrocarbon compounds – also known as fractions – that the oil contains. To do this, the refining process makes use of the different boiling points of each fraction.

The diagram below shows the distillation process. This process can be broken into two basic steps: atmospheric distillation and vacuum distillation.

Atmospheric distillation

The crude oil is heated to around 370°C at atmospheric pressure (1). This causes it to evaporate. The vapor is then pumped into a fractionating column (2). This column varies in temperature: The bottom, where the crude oil enters, is very hot, but the column becomes progressively cooler towards the top. The column is divided throughout its height by fractional trays (3). These are perforated platforms fitted with bubble caps. The vapor must pass through each platform as it rises through the column, causing it to cool down and recondense. However, due to their different boiling points, each fraction recondenses at a different level, where it can be drawn off. This results in each fraction separating from the others (4).

In order to prevent the fractions with the lowest boiling points from degrading or “cracking,” the temperature they are heated to cannot exceed 400°C. This, however, means that not all hydrocarbons can be separated, as some have even higher boiling points. A residual mixture of these therefore remains after distillation (5). To separate this residue, known as atmospheric residue, it is distilled a second time, this time under vacuum.

Vacuum distillation

Under vacuum, the boiling point of liquids is reduced. As some hydrocarbons have a boiling point of nearly 500°C, this is extremely beneficial. Subjecting all compounds in the atmospheric residue to such high temperatures would result in the lightest breaking down before the heaviest have vaporized, causing unwanted byproducts. By lowering the pressure, the residual hydrocarbons can be further separated at only slightly higher temperatures.

In the rough vacuum range, between 10 to 50 mbar, the residue is distilled a second time in the vacuum distillation column (6). Just like under atmospheric pressure, the different fractions recondense at different levels and are drawn off (7).

Learn more about crude oil distillation

Which Busch vacuum solutions are especially suitable for crude oil distillation processes?

For a wet and hazardous process with a high flow requirement like crude oil distillation, DOLPHIN liquid ring vacuum pumps from Busch are the ideal choice. Busch experts will be happy to assist you in finding the vacuum pumps that are the best match for your process.

What type of distillation is used for crude oil?

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.

What is the importance of crude oil distillation?

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.

What is the boiling point of crude oil?

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.

What is vacuum distillation of crude oil?

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.

What are the components of crude oil?

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.

What is cracking?

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.

What level of vacuum is used for crude oil distillation?

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.

What is the main purpose of crude oil?

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.

What is the temperature range of crude oil distillation?

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.

What is the difference between crude oil and petroleum?

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.