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How does vacuum distillation process heavy petroleum materials without causing thermal cracking?

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Aerial view of an oil refinery complex near New Orleans, Louisiana, with storage tanks, industrial structures, and smoke rising from the facility, surrounded by open land. — petroleum refining Aerial view of an oil refinery near New Orleans.

Vacuum distillation allows for the processing of heavy petroleum materials by significantly lowering the temperature required for distillation, which prevents the unwanted thermal cracking of sensitive components.1

In standard fractional distillation, high temperatures are necessary to vaporize heavy hydrocarbons. However, these high temperatures can trigger thermal cracking, a process where large molecules are broken down into smaller ones, often resulting in the formation of coke and other undesirable by-products.23

Vacuum distillation avoids this by operating at reduced pressures, typically between 50 and 100 mm of mercury absolute, which are maintained by vacuum pumps or steam ejectors. Because the boiling point of a substance is related to the pressure of its surroundings, reducing the pressure allows the heavy materials to vaporize at much lower temperatures—usually keeping oil temperatures below 425 °C (800 °F). By maintaining these lower temperatures, refiners can effectively separate heavy fractions, such as those used for manufacturing lubricants, without the chemical degradation caused by thermal cracking.14