Distilling heavy fractions like atmospheric residue under vacuum at reduced temperatures is necessary because high temperatures promote significant____________________?
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Answer: C. Coking
Explanation
Exposing heavy distillation residues to high temperatures without vacuum causes severe thermal coking. Thermal coking deposits hard carbonaceous material inside furnace tubes and ruins distillation equipment. Lowering the system pressure under vacuum allows vaporization at safe temperatures to prevent coking. Atmospheric residue contains massive asphaltic molecules that break down rapidly when overheated. Coking involves the polymerization and condensation of heavy hydrocarbons into solid carbonaceous coke. Vacuum distillation lowers boiling points significantly below the thermal decomposition temperature threshold.
Refinery vacuum towers prevent costly unscheduled shutdowns caused by severe furnace tube fouling. Maximizing heavy distillate recovery improves overall crude oil processing economics considerably. Proper operation protects downstream catalytic units from heavy metal and carbon contamination. Option A refers generally to cracking while coking specifically describes the severe solid carbon buildup. Option B involves gum formation which typically occurs in lighter gasoline storage rather than hot residues. Option D names discoloration which is a minor aesthetic issue compared to catastrophic unit coking.
- Subject
- Engineering
- Category
- Chemical Engineering
- Topic
- —
- Difficulty
- medium
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