Identify the non-exothermic process: Which of the following industrial processes is not exothermic overall?

Introduction / Context:
Thermal effects determine reactor design, heat removal or supply, and safety. Many large-scale syntheses are exothermic and require heat removal, while some refining steps are endothermic and need continuous heat input.

Given Data / Assumptions:

• Consider net reaction enthalpy at typical process conditions.
• Processes listed are well-known in petrochemicals and bulk chemicals.

Concept / Approach:
Catalytic cracking breaks large hydrocarbon molecules into lighter products (e.g., gasoline-range). Breaking C–C bonds without compensating hydrogen addition is strongly endothermic; fluid catalytic cracking (FCC) regenerators burn coke to provide the heat duty for the endothermic riser reaction. In contrast, methanol and ammonia syntheses and sulfur oxidation are exothermic, releasing substantial heat.

Step-by-Step Solution:
List heats of reaction qualitatively: methanol synthesis → exothermic; ammonia synthesis → exothermic; sulfur oxidation → exothermic.Catalytic cracking → endothermic; heat supplied via hot regenerated catalyst and regenerator flue gas heat integration.Therefore, the only non-exothermic option is catalytic cracking.

Verification / Alternative check:
Plant energy balances show large heat removal in syntheses (reactor coolers), versus heat input in cracking (hot catalyst circulation, fired heaters where applicable).

Why Other Options Are Wrong:

• Methanol and ammonia syntheses: both release heat per stoichiometry and require temperature control to maintain equilibrium conversion.
• Sulfur oxidation: strongly exothermic; exploited in sulfuric acid plants.
• Hydrogenation of oils (not listed in the original options if omitted) is exothermic as well.

Common Pitfalls:
Confusing catalytic cracking with catalytic reforming/aromatization; both are endothermic but serve different refinery objectives.

Final Answer:
Catalytic cracking of heavy hydrocarbons