Thermal effects in refining reactions: Select the clearly endothermic reaction (dominantly positive heat of reaction under typical refining conditions) from the following list.

Introduction:
Process heat effects determine reactor design, heat integration, and catalyst life in refineries. Some conversions absorb heat (endothermic), while others release heat (exothermic). This question asks you to identify the reaction that is unambiguously endothermic under standard refining conditions.

Given Data / Assumptions:

• Representative reactions: cracking, hydrocracking, dehydrogenation, polymerisation, alkylation.
• Typical refinery temperatures and pressures are assumed.
• We focus on the sign of the heat of reaction (endothermic vs exothermic).

Concept / Approach:
Breaking C–H bonds and removing hydrogen (dehydrogenation) requires energy input, making the reaction endothermic. In contrast, reactions that create more saturated products (hydrogenation components of hydrocracking and alkylation) are exothermic. Polymerisation of olefins releases heat as new C–C bonds form. While cracking steps often require furnace heat input, the reaction list includes one case that is thermochemically definitive: dehydrogenation of naphthenes to aromatics (e.g., cyclohexane to benzene + 3 H2) is strongly endothermic at reforming conditions.

Step-by-Step Solution:

Verification / Alternative check:
Reforming textbooks emphasise external firing and high-severity heat input to drive dehydrogenation/aromatization; hydrogen generation is an indicator of endothermicity.

Why Other Options Are Wrong:

• Catalytic cracking: Requires heat input but often considered endothermic with complex side reactions; however, among the listed, dehydrogenation is the most clear-cut endothermic case.
• Hydrocracking: Net exothermic due to hydrogen addition.
• Polymerisation: Exothermic from C–C bond formation.
• Alkylation: Exothermic condensation of isoparaffin with olefin.

Common Pitfalls:
Assuming any process that “needs a furnace” is the clearest endothermic choice; thermochemical certainty is strongest for dehydrogenation in this list.

Final Answer:
Dehydrogenation of a naphthene to form an aromatic