Explain the relationship between the degree of vacuum applied in a vacuum distillation unit and the prevention of thermal cracking of the residue.

The degree of vacuum applied in a vacuum distillation unit is directly related to the prevention of thermal cracking of the residue. A vacuum distillation unit operates under significantly reduced pressure compared to an atmospheric distillation unit. The purpose of applying a vacuum is to lower the boiling points of the heavy hydrocarbon components in the residue, allowing them to vaporize at lower temperatures. Thermal cracking is the breakdown of large hydrocarbon molecules into smaller ones due to excessive heat. If the vacuum is insufficient, higher temperatures are required to vaporize the residue, which increases the risk of thermal cracking. This is because the boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding pressure. By reducing the pressure, the required temperature to reach the boiling point decreases. Thermal cracking is undesirable in vacuum distillation because it can lead to the formation of coke, light gases, and lower-value products, reducing the yield of valuable vacuum gas oils (VGO) and increasing operational problems like fouling. Maintaining a high vacuum, typically in the range of 10-40 mmHg (millimeters of mercury), ensures that the residue can be distilled at temperatures below the threshold for significant thermal cracking, usually around 700-750°F (371-399°C). The effectiveness of the vacuum is crucial; a small improvement in the vacuum can significantly reduce the required operating temperature and minimize thermal degradation. For instance, if the vacuum is not sufficient to vaporize the heavy components at safe temperatures, the residue may undergo cracking, producing lighter hydrocarbons and coke, which can foul the column and reduce its efficiency. Steam ejectors or vacuum pumps are used to maintain the desired vacuum level. The proper functioning of these systems is essential for achieving optimal separation and preventing thermal cracking. Careful monitoring and control of the vacuum level and the furnace outlet temperature are necessary to balance product yields with the risk of thermal cracking. Therefore, a higher degree of vacuum allows for lower operating temperatures, minimizing thermal cracking and maximizing the recovery of valuable products from the residue.