ANS:C - coking

Higher boiling fractions like atmospheric residue are distilled under vacuum at low temperature because at high temperature, there is a tendency of the predominance of thermal cracking.

Explanation:

• Atmospheric Residue: This is the heavy fraction that remains after the atmospheric distillation of crude oil. It contains high-boiling point components that require further processing to be converted into valuable products.
• Vacuum Distillation: This process involves distilling the atmospheric residue under reduced pressure (vacuum), which lowers the boiling points of the components. This allows distillation to occur at lower temperatures.

Reasons for Using Vacuum Distillation:

• Thermal Cracking: At high temperatures, large hydrocarbon molecules in the atmospheric residue are prone to breaking down into smaller molecules through thermal cracking. This can produce undesirable products and degrade the quality of the distillates.
• Other Issues at High Temperature:
  • Gum Formation: Some heavy components might form sticky substances that can foul equipment.
  • Coking: High temperatures can lead to the formation of solid carbonaceous deposits (coke), which can block equipment and reduce efficiency.
  • Discoloration: High temperatures can cause the hydrocarbons to undergo reactions that lead to discoloration of the product.