Oil Refining Distillation Towers Explained

What are Distillation Towers in Oil Refining?

Crude oil is a complex mixture of hydrocarbons. While it is a form of oil, usable petroleum products are obtained after specific physical and chemical processes. A primary method for separating crude oil is fractional distillation. This method separates crude oil into different fractions and petroleum products, although not always in a pure state initially. This process is carried out in large industrial complexes known as refineries. Refineries subject crude oil to physical separation processes using a large variety of industrial distillation towers. These towers can contain numerous trays, separating crude oil into various compounds that are further processed to yield around 2,000 different products.

Refineries and Atmospheric Distillation

Refineries are composed of tall distillation towers. The primary distillation tower, often called the atmospheric distillation tower, works at atmospheric pressure. These towers have many internal compartments or trays, each operating at different temperatures. This temperature gradient ensures that each tray serves a specific function in separating different hydrocarbon fractions.

Atmospheric Distillation Process

Crude oil is first heated in a furnace (up to 400 °C), turning much of it into vapor. This mixture of liquid and vapor then enters the atmospheric distillation tower near the bottom. The vapors rise through the tower, cooling as they ascend. As the vapors cool, different hydrocarbon fractions condense into liquid at different temperature levels, collecting on the trays. Lighter fractions (lower boiling points) rise higher before condensing, while heavier fractions condense lower down. The remaining liquid that does not vaporize (known as reduced crude or residue) collects at the base of the tower.

Vacuum Distillation Towers

Vacuum distillation towers are designed to operate under reduced pressure (vacuum) to distill the heavier fractions of crude oil that did not vaporize in the atmospheric tower. Operating under vacuum lowers the boiling points of these heavy hydrocarbons, allowing them to vaporize and separate without reaching temperatures high enough to cause thermal decomposition (cracking). Vacuum systems, such as steam ejectors, are used to maintain absolute pressures as low as 20 mm Hg in the column.

Why Vacuum Distillation is Necessary

Vacuum distillation is a complementary operation to atmospheric distillation. It processes the heavy residue (reduced crude) that remains at the bottom of the atmospheric column. Distilling this residue at atmospheric pressure would require temperatures high enough to cause thermal decomposition, which is undesirable at this stage of refining as it can damage equipment and produce unwanted byproducts.

Atmospheric Distillation Conditions and Output

In the atmospheric distillation tower, crude oil is typically preheated and desalinated using heat recovered from other process streams. It then passes through a direct-fired heater before entering the distillation column just above the bottom. The column operates at pressures slightly above atmospheric and temperatures typically between 343 °C and 371 °C at the feed point. These conditions are carefully controlled to avoid thermal cracking. The lighter fractions (those with lower boiling points) vaporize and rise to the top of the tower, where they condense and are continuously withdrawn. These light fractions are then sent to other refinery units for further processing, blending, or distribution.

Principle of Reduced Pressure Distillation

Distillation is a process designed to separate liquids based on their boiling points. For certain heavy hydrocarbon fractions obtained from crude oil, their boiling points at atmospheric pressure are very high. Attempting to distill them at these temperatures would cause the material to decompose or crack before it vaporizes. For example, hydrocarbon chains begin to decompose around 350 °C, a temperature that might be below their atmospheric boiling point. The solution is reduced pressure distillation (also known as vacuum distillation).

Reduced pressure distillation involves lowering the pressure under which the liquid is distilled. Normal atmospheric pressure is approximately 760 mm Hg. By reducing the pressure significantly (e.g., to around 40 mm Hg as mentioned, or even lower), the boiling points of the heavy hydrocarbons are also significantly reduced. This allows the material to vaporize and be distilled at much lower temperatures, below the point where thermal decomposition occurs.

Stripper Towers

Stripper towers are typically smaller towers whose primary function is to remove lighter, low molecular weight components (more volatile) from liquid side streams drawn from larger fractionating towers. The physical principle behind their operation is to decrease the partial pressure of the lighter components in the liquid by injecting a stripping medium, usually steam or dry gas, into the bottom of the tower. As the stripping medium rises through the liquid, it helps vaporize and carry away the lighter hydrocarbons. This process helps to adjust properties like the flash point of the fuel products.