Operating pressure in cracking units: Although high pressure tends to retard cracking, industrial units still maintain a positive pressure of about 10–15 kgf/cm^2 mainly to achieve what?

Introduction:
Cracking reactions form lighter products at elevated temperatures. Pressure influences phase behaviour, secondary reactions, and coking tendency.

Given Data / Assumptions:

• Typical operating pressure: about 10–15 kgf/cm^2.
• Cracking rate itself is retarded by higher pressure.

Concept / Approach:
Moderate positive pressure helps keep heavier material in the liquid phase, improving heat transfer and reducing the tendency to form coke on reactor surfaces. Lower gas holdup also curbs overcracking and polymerization leading to coke.

Step-by-Step Solution:

Verification / Alternative check:
Industrial practice balances temperature (to achieve conversion) with pressure (to limit coking and gas make), selecting ~10–15 kgf/cm^2 as a compromise.

Why Other Options Are Wrong:

• Increase light distillates: Pressure alone does not increase yield; temperature-severity is primary.
• Enhance octane directly: Octane relates to product composition, not directly to pressure setting.
• Reduce gum content: Gum control is more related to olefin management and treating.
• Raise temperature without runaway: Temperature control is independent of maintaining moderate pressure.

Common Pitfalls:
Assuming any operating change that stabilizes operation must also increase octane or light-end yield.

Final Answer:
Suppress coke formation and maintain liquid-phase operation